Phil's Superpower of Enthusiasm

[ORIGINAL MSRP $499.99, available for $399.95 at the Sennheiser store on Amazon. Borrowed from a friend for review in September 2025.]

[Tl;dr: The Sennheiser 660S are a strange product, representing in a lot of ways a step backwards from the excellence of the Sennheiser HD 600/650 and the Sennheiser x Drop HD 6XX, and because of their price point, they end up without a strong place of their own. They’re a perfectly pleasant headphone, but for my standards they’re beaten pretty handily by the other members of the 600 series and most of the comparables in my collection. If you can find them at a discount or lightly used, they’re a good headphone. New, at or anywhere near MSRP? There are much better deals out there in my opinion.]

Scores:

Cost-agnostic: 6 out of 10 Denalis

Cost-sensitive:  4 out of 10 Denalis

1. Intro to the intro: Quick Reviews!
2. Intro
   1. Review notes
3. The basics
4. Overall impressions
5. Sound
6. Build
7. Value/Comparisons
   1. HD 600/650/6XX
   2. Beyerdynamic DT 990 Pro
   3. Fiio FT1 Pro
   4. Hifiman XS
   5. Meze 105 AER
8. Overall

Intro to the intro: Quick Reviews!

As introduced in the iFi Uno review, I’ve decided to lean into a new feature here at Phil’s Superpower of Enthusiasm, Quick Reviews! There are a number of things that I’d love to get quick notes down on for my own edification/memory, but that I don’t want to spend the 10-15 hours I devote to most of my (overly?) in-depth reviews. Some of these will be things that aren’t in production anymore (so it’s less likely anyone will read a review), or are extra niche, or are things that I didn’t particularly like but want to be able to point people to my reasoning, or that are in a category I don’t spend a lot of time with (like IEMs).¹ Today: the Sennheiser 660S!

Intro

I have have long been a fan of Sennheiser’s headphones; my first pair of over-ears was a old 500 series when I was in middle school, and my first real pair of audiophile grade headphones was the truly excellent Sennhesier x Massdrop (now just Drop) 6XX. I’ve owned at least three sets of Sennheiser RS175s, the original (wired) Momentum, the HD 515, and two pairs of Momentum 4s as well as a couple of their wireless earbuds. A few months ago I snagged a pair of HD 600 on Prime Day, and I’ll likely have a review of those in the near future.

I like the 6XX enough that they continue to be a regular listen for me, despite the fact that I own some much more expensive headphones (even some that I think sound substantially better); there’s both a real nostalgia factor with the 6XX as well as a frequency response that I really love for most of the music I listen to (clean but not emphasized bass, beautiful mids, mellow treble, excellent reproduction of silence, etc.), but I’d never really bothered to try the other modern Sennheisers other than casually demoing a few over the years (599, 560S, 58X, etc.) because none seemed competitive with the 6XX.²

Enter my friend Dillon; after borrowing a couple of my Hifimans and the Fiio FT1 Pro, Dillon offered me a chance to borrow and review his daily driver, the HD 660S (and a Hifiman Sundara, review forthcoming).

Review notes

Terminology: I’m going to be referring to the HD 600/6XX/650 family a lot in this review, and I will refer to them collectively as the 6X0. General modern consensus seems to be that the differences between these three models (particularly between the almost identical 650 & 6XX) may be smaller than the unit variation within each model, which largely comports with my own experience with the 600/6XX in my collection; I can hear some consistent but minor differences, but I don’t know if that’s because of the design differences or just my specific units.

Testing rig: Based on my philosophy on the allocation of resources in headfi, except where otherwise noted I’m going to primarily be testing these with a Schiit Modius/Magnius stack, running balanced via Hart interconnects and connected via USB-C to a custom gaming PC running the Roon client.

EQ: As noted in the Bathys review, I mostly prefer to test headphones with their default tuning, assuming that most people won’t take the time or make the effort necessary to use a third-party EQ. And like most Sennheiser 6-series products, these don’t need EQ, though they may benefit from a bit of tweaking on the margins.

Volume: Here are the volume settings I use with the Sennheiser HD 660S (unless otherwise noted, running via Roon with no headroom management, playing Daft Punk’s “Face to Face”):

• Mimir/Jotunheim 2 (Mimir has a -15 dB pregain applied via Forkbeard): low gain, 11:00 (balanced); low gain, 12:15 (single-ended)
• Modius/Magnius: low gain, 11:00 (balanced); low gain, 11:45 (single-ended)
• Modi/Magni: low gain, 9:00
• E30/L30: lowest gain, 12:00
• Chord Mojo: white, light red, dark red, dark red³
• North American Apple dongle via iPhone 15: 85%
• Apple dongle via Mac Studio: 75%

My torture testing list: Apple, Tidal, Spotify.]

The basics

The Sennheiser HD 660S is a passive, open-back headphone in Sennheiser’s venerable 6-series. It was released in 2017 as an update to the HD 650, to generally positive reviews at the time.⁴ It’s a moderately difficult-to-drive headphone, at 150 Ohms of impedance and a sensitivity of 104 dB/mW. The HD 660S recieved an update in 2023, the HD 660S2. The 660S is no longer in production, though it is still widely available both new and used.

Overall impressions

The 660S feel a little like a product in search of a niche. The 600/650/6XX are such a good product and are still widely available⁵ (and the 6XX is such a strong value proposition) that a new product in this series would have to be a substantial improvement to draw much attention. This is especially true given the 660S’ aggressive price point of almost $500, in a world in which the 6XX routinely cost less than $200 and the 600/650 are often on sale for less than $300 and around $350 respectively.

They’re not a bad product; they just don’t differentiate themselves enough for me to prefer them over the 6X0 family, and I’m not surprised that Sennheiser updated them pretty quickly with the 660S2.⁶

Sound

The 660S are a 600 series headphone, which means that they provide pretty good resolution, detail retreival, and vocals. They purport to be more natural and easier to drive than the 650. One of these things is more true than the other.

The soundstage/stereo separation is pretty good left to right; the placement in space is consistent and reasonably wide but doesn’t have much depth, front-to-back. The toms on “Thunder Lightning” track beautifully left to right. The dynamics are pretty good; both the bell tones and the treble chimes in “Angel (Blur Remix)” are solid and well produced (perhaps even a hair more clearly than the HD 600).

Bass is clean and clear; not unlike the 6X0. It’s not particularly pronounced or recessed, just … there. The cello on “What Did I Do?” is beautifully reproduced, if maybe a slight bit less rich and warm than on the 6XX in particular. I do find some tracks (like “Moddenaminute” by Smino) to be a little splattery/thumpy on the 660S, though that may be a function of this specific unit rather than the model as a whole.

Mids are really quite good, both for instruments and vocals, with vocals perhaps even a bit more forward than on the 6X0. Jill Scott and Justin Hicks are both nicely reproduced with excellent, natural timbre.

The treble is where the 660S sort of lets you down; at least to my ears, it feels like it’s missing a bit from the treble response, and in a way that reduces at least the perception of detail. There’s also a funky warble on some tracks (most noticeably on tracks like Mac Miller’s “Stoned”, where a panning treble tone can be VERY distracting when it hits certain frequencies).

The 660S are easier to drive than the 6X0, a welcome change, and seem to not be quite as amplifier sensitive as the 6XX, at least

Build

For this quick review, I’m just going to refer you to the build section of my HD 6XX review; the only noticeable difference in the build qualitiy between the 6XX, the 600, and the 660S is the raised Sennheiser logo and the curved sections on the grills. Otherwise the three are basically identical in their build other than the different model numbers on the temples, of course, and the colorway.

Value/Comparisons

HD 600/650/6XX

The most obvious comparisons for the HD 660S are the headphone it was designed to replace, the HD 650,⁷ and the headphone that the HD 650 was designed to be an upgrade for, the HD 600. Fortunately, all three are sitting on my desk!

Amplification. The first point I’ll make is that the HD 660S are, in fact, easier to drive than either of their older cousins. At 150 Ohms and 104 dB/mW, it has half the impedance of both the 600 and the 6XX, while sitting right between them in terms of sensitivity. I personally think the difficulty of driving the 6XX (and to a lesser extent the 600) is MUCH exaggerated on the internet, but to the extent someone is having a problem driving either, the 660S may be an easier headphone for them to use with their setup.

Sound. I’ve said before, and after a bunch of A/Bing I stand by it, for me the audible differences between the HD650/6XX and the HD600 are minimal, and I’m absolutely open to the idea that the differences between the models may be smaller than the unit variation within a particular line.

First, the elephant in the room: like most headphones, the Sennheiser 600 series is pretty sensitive to pad wear. The frequency response (and comfort!) can vary a lot depending on how worn in the pads are. Both the 6XX and the 600 I’m testing have fresh pads (the 6XX were replaced a month ago and the 600 was purchased, new, a month ago) while the 660S are the original pads so have probably experienced a couple of years of regular use.

With the specific units that I have in my house for this review, here are my thoughts:

• The bass is slightly stronger on the 6XX than the 600, and the 600 is stronger than the 660S (in terms of volume AND quality).
• The mids are substantially more forward on the 660S than either the 6XX and the 600, in a way that does not make me prefer them but is NOT so exaggerated as to be unpleasant.⁸
• The 660s has a substantially backed off treble performance compared to both the 6XX and the 600, in a way that impacts *at least my perception* of detail retrieval. I find the treble on the 600 and 6XX largely interchangeable, though I very occasionally hear a sharpness in some frequencies on the 600 that I don’t find in the 6XX (and I very occasionally also think I hear more detail in the 600, potentially a hallmark of stronger treble performance). I don’t particularly like the treble presentation on the 660S; it ends up feeling a bit hollow and tinny at times.

So, now that I’ve talked about my subjective impressions, let’s look at Super* Reviews’ graphs! Drum roll ….

Oh, good.⁹ So this largely matches with my subjective experience; the 660S does have reduced bass response¹⁰, stronger mids, and lower treble performance than either the 6XX and the 600. The 600 has lower bass and mids response than the 6XX, and a little more treble. All three sit underneath Super* Reviews’ baseline target when it comes to the bass, but we more or less knew that would be true given all of the complaints about bass performance across the series as a whole.

Just because of how much variation the 6X0s can experience with worn pads, let’s look at another venerable headphone review institution’s graphs:

It’s always nice when two different organizations using two different machines come up with roughly the same data.

For me, I come away preferring the sound of the 600 and the 6XX over the 660S. Between the 6XX and the 600, it’s a much closer call and might depend on what I’m listening to; I’m generally less a treble fan than most and in a vacuum I like a little more bass, but the thing about the 600 is that the bass is still good, and the treble manageable, and I might be willing to make that trade for a little more perception of detail. I’m going to spend some more time with both this month after the 660S goes home, but at this point I’m pretty happy with either.

Price independent: my initial thought is 600 > 6XX > 660S, but this may be a little bit of my subconscious wanting a reason to keep the 600 around.

If you consider the price? The 6XX is the clear winner. At it’s regular price of $199, the 6XX is an absolute steal for value. When it’s on sale around $180? Even better. While there are tracks that I will prefer the 600 on, to me the better treble wouldn’t justify the additional cost. And because I don’t think that there are any tracks I would prefer the 660S for, Sennheiser’s pricing puts them out of consideration for me.

Don’t get me wrong; the 660S are still a good headphone, they’re just up against two headphones that are classics for a reason. I’m curious to source a pair of 660S2 at some point to do the comparison again; I’ve heard that they actually represented a step up.¹¹

Beyerdynamic DT 990 Pro

I really liked the Beyerdynamic DT 990 Pro back when I first started getting into headfi. The consensus at the time was that the 990 was a great studio-style open-backed headphone. The consensus was … well, wrong, and I hadn’t heard enough good headphones to know any better myself.¹² I probably ought to do an update to that review before I sell the 990s … I’ll add that to the list.

Here’s the short version: the 990 have excellent soundstage, great comfort, reasonably good build quality, pretty good layer separation, good mids (especially the human vocal range, especially with people talking normally), and a wonky-ass tuning that makes those strengths pretty much irrelevant to me. Here’s Cammyfi’s graph comparing the 990 against the 660S:

I don’t mind the subbass; it’s a good solid thump. The first problem is the mid-bass, which is such a weird bump that it starts to really impact the mids in ways I find distracting and unpleasant, and then that spikey treble means that like 10% of songs sound pretty awful to me. Again, Rule #1, but the 990 are so not for me. The 660S is a much better pick for me, even at almost 4X the price, if those were the two options.

They’re not, though.

Fiio FT1 Pro

This is an interesting comparison; in my FT1 Pro review I compared the Pro to the 6XX pretty extensively, and while I found that the 6XX is better for MY personal listening, it was pretty close. I preferred the 6XX’s mids and ability to play quietly, while I thought the Pro had a slight edge on bass and treble presentation on some tracks. I look forward to doing some critical listening between the Pro and the HD 600 at some point soon.

As far as the Pro vs. the 660S, I think I give the edge to the Pro here. I think they’re pretty even when it comes to sound quality (with the Fiio running away with bass, losing on mids, and being an interesting but ultimately toss-up mixed bag on the treble), but with the Pro running more than $100 less than the 660S, I’m going to give the edge to Fiio. If I were listening only to mid-centric, mostly vocal music, I’d prefer the 660S; for everything else I listen to (aka 90% of my listening), the Pro are a better fit for me. These impressions are backed up by the graph:

Hifiman XS

While I preferred the 6XX over the Edition XS because of the more laid-back treble performance, the 660S might go so far in managing treble as to ultimately lose out to the Edition XS for me (though I much prefer mids and vocal range on the 660S, the XS’ bass response is also worlds better). As much as I’m pissed at Hifiman’s customer service and general attitude towards their customers,¹³ they make REALLY good products, and really good products for value especially after the under-the-table late 2024 MRSP reset. While I would always encourage everyone to buy from a retailer with a good return policy that won’t make you EVER deal with Hifiman directly instead of buying from Hifiman direct,¹⁴ all else being equal I think I’m buying the Edition XS over the 660S and applying some EQ to tame the occasionally wild treble.

Super* Review’s graph here:

Good lord that treble is spicy. But, ultimately, not super hard to EQ, and it’s worth it for the substantially better bass (and particularly sub bass).

Meze 105 AER

In my review of the Meze 105 AER, I compared the AER to the 6XX directly and came down largely on the side of the AER (except for some vocal-centric music). I think it’s not super surprising, then, that I’m going to give the clear win to the AER here. The AER have better bass and treble performance (at least to my personal preferences, with the exception of a couple of slightly weird spots in the treble), are pretty competitive in the mids, and are one of the most comfortable headphones I’ve ever worn. Given that they’re retailing around the same price point right now, I’d definitely prefer the AER over the 660S.

Super* Review’s graph largely aligns with those perceptions:

Overall

I started with this, and I’ll end with it too; the 660S feels like a product in search of a niche. It’s not a bad headphone, but to me its a distinct step backwards from the excellent HD 600/650/6XX, and because of that the price disparity seems … odd. If they had made improvements in exchange for some of the downsides, or priced the 660S under its predecessors as a budget option, it might be a different story, but for me, I’m just never going to pick them over a half dozen things in my collection so they’re not for me.

#reviews #headphones #sennheiser #6XX #anc #spatialaudio #meh #2025 #99noir #meze #sunglasses #overear #cans #hifiman #arya #stealth #editionxs #budget #hahahaha #iems #quickreview

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1. Though I still aim to not spend much time or brainpower on things I don’t like, generally. Sometimes it’s nice to be able to copy/paste when someone asks me why I don’t like or recommend a thing. ↩︎
2. I’ve only briefly listened to the 800 and 800s, and at least at first glance … neither are for me. I have borrowed a pair of the Sennheiser x Drop HD 58X for review in the near future. ↩︎
3. The Mojo shows volume with three lights: the first (effectively) shows you the gain setting and the other two a color-coded progression from quiet to loud. ↩︎
4. Editor’s note: looks like like modern reviews are … less kind. ↩︎
5. I think all three are still in production as of 2025? ↩︎
6. Never had a chance to spend any meaningful time with the 660S, but if you live in the Puget Sound region and want to give me a chance to listen to them … or hey, Sennheiser, want another review? ↩︎
7. And of course the 6XX is just an older chassis and a new colorway but sound-wise identical to the 650. ↩︎
8. For me, the 6XX/600 hit the sweet spot for natural vocal performance and balance. The 660S just turns it up to eleven. ↩︎
9. Whenever I go to the graphs, I live in fear that I’m going to discover that my hearing is totally shot (or that I have spent so much time and energy reviewing a non-standard or faulty unit). ↩︎
10. I guess slightly better subbass response, but that’s a range I mostly feel rather than hear anyway. ↩︎
11. Hey, Sennheiser, I’ll do a fair review if you send me a unit! ↩︎
12. As always, observe Rule #1: you like what you like, regardless of what I think (or anyone else thinks) about it. You’re allowed to be wrong. 🤷‍♂️ ↩︎
13. More to come here, as soon as my issue gets resolved or I decide to give up. ↩︎
14. And dear god, don’t buy open-box or refurbs from them … ↩︎