Review: Sennheiser HD 660 S (With HE-400I Comparisons)

Sennheiser HD 660 S
Sennheiser HD 660 S

So my old AKG K271 MKII was starting to show their age, and with Christmas, I decided it was time for an upgrade.

I ended up with choosing between the HIFIMAN HE-400I and Sennheiser HD 660 S, as the HE-400I was on a big discount which put them in the same price bracket.

I got the Sennheisers setup at the store, with some fancy equipments that are way out of my price league, and went to town. I put on my best “critique” impression – writing things down on paper so that I could compare, going through jazz, neo-soul, choir pieces, pop, orchestral, the whole shazam!

I was enjoying myself immensely with the detail of the 660s (more on that later), while still focusing on all the “bad” things with the headphones.

Then I changed to the HE-400I. And it just couldn’t compete.

The soundstage was worse, and the instruments didn’t sound as good; like they were wrapped in wool. Now at this point, I had been listening for an hour and a half to the 660S and was thinking I didn’t remember the sound of the first tunes properly. So I decided to switch between the headphones and listen shortly to each tune, but no.. the 660S were more open and realistic compared to the HE-400I.

Now I admit that the HE-400I might be a bit more “fun” on some of the tunes with a lot of bass, (and I’ve probably got buyers bias) but the price you’d end up paying on acoustic tunes was not worth it to my ears. So I brought them back to the counter and left much poorer, but happier, clutching the 660S to my chest.

What We Like 😍
  • Very detailed
  • Warm mids
  • Great vocals
  • Medium soundstage
  • Good and clear separation between instruments
  • No muddiness
  • Instruments sounded natural and open
  • Great for acoustic music

What We Don't Like 🤢
  • Lacks just a bit of bass at the bottom
  • Can be a bit harsh on the highs if you’re sensitive to that
  • Needs good source material (both in file size, mixing and recording) to truly shine.
  • Weak when it comes to the heaviest electro
  • Not super smooth

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Build Quality and Comfort

The matte black exterior looks good and gives an understated and muted look to the headphones. They’re sturdy, and the build quality feels good.

The cables seem to be of high quality, and they include an adapter for the 3.5 mm stereo jack. They do also include a balanced 4.4 mm jack. I, however, don’t have any equipment available to test whether it affects the sound quality. A complaint is the 3m long cable seems excessive. I personally use my headphones with the computer and would like a coiled or shorter cable to be included. They seem to be marketing these to be used with smart devices, so the longer cable seems a bit misplaced.

My old AKG with my Sennheiser HD 660 S
My old AKG with my Sennheiser HD 660 S

They are, however, very comfortable. The headphones are snug against the head, without too much clamping force, and quick movements don’t displace them. The paddings are also not too soft, and when adjusting the headband, there’s a satisfying click that lends itself to the feeling of quality.


I’ve read that the 660S’s soundstage described as medium-large compared to the HD 800’s, but since I’ve never listened to the latter, I can’t compare.

Its soundstage is however large enough that you’re able to separate the instruments, and they have enough room and separation to sound real. From the plucking of the string on the double bass, the timber of brass and strings, to the strike on the cymbal; they all sound natural and distinct in the soundstage.

Sound Quality

Now; I listen mostly to jazz and acoustic music. I’m also a jazz pianist and so I’d say that I’m somewhat familiar with how instruments sound live. I must say the detail that the 660S delivers is really good.

The vocals are also great, with warmth and enough highs to sound open. The bass sounds tight and deep and is not overpowering. And even though they do a good job with acoustic music, they’re not chumps with electronic or rock.

D’Anglo sounds great with the deep bass, and I catch myself bobbing along to Moonchild, Hiatus Kaiyote, Stevie Wonder.

They are, however, quite unforgiving. Bad quality shows really quickly; Bernhoft’s Ceramic City Chronicles is one of my favorite albums, but on the 660S it doesn’t sound as good.

It’s not a badly mixed album, and that is in lossless format, but the small deficiencies are noticeable. Bad recordings and poor playing are very noticeable with these headphones. On the flip side, the great albums never sounded so good.

Detailed Sound

I’ve seen a lot of reviews that compare them to the HD600 and HD650. I haven’t listened to those, but they’re described as smooth, if not a bit muddy/veiled. Instead, I would describe the 660s as detailed.

I heard a lot of new things when going through my music. While listening to Hiatus Kaiyote’s Choose your Weapon, I was able to separate the vocal overdubs and noticed synths that were in the background. Usually, I find that the lower-mid vocals disappear into the mix when using headphones, but here they were clear and distinct. The instruments also sounded more natural, with the plucking of strings coming through clearly.

Lack of Bass

On their track Swamp Thing, the rhythm of the bass in the intro was much more defined, but it lacks the same fullness and grittiness that I have with my (much worse) AKG. I think this track shows the lack of bass in the 660S, and how that may be off-putting for some.

Another example is Flying Lotus’ Tea Leaf Dancers, a track that needed that powerful bass to make the sidechaining pop. This is where other headphones would outperform the 660s. That’s not to say that the bass is awful. I found it more than satisfying on all the music I listened to by Stevie Wonder, D’angelo, Earth, Wind & Fire, Anomalie, Moonchild, and Hiatus Kaiyote.

Also, the drop on KNOWER’s The Government Knows put a stupid grin on my face. 

For me, I value the precise timing more than the fullness. I do expect that a small bit of EQ on the lower bass would address this problem with the headphones. I also find that the definition of the bass makes up for it, with it sounding less murky and more defined. It ends up sounding not just like bass, but rather shows the instruments own character (as it should be).

Source of material matters

I also found that on some tracks it would get almost shrilly, especially in the horns on I Wish by Stevie Wonder. On some of the classical pieces, the mid-highs could get a bit overpowering, but switching to another recording of the same piece and that part would sound great.

Here again, we see that the source material needs to be good; any deficiencies shines through. And when it’s good; daaaaaaaaaaaaamn. Birds by Marius Neset is a treat, as is Eroiki by Trondheim Jazz Orchestra and Joshua Redman.

The Fiio as well. Not the best DAC/AMP, but it does its job well (and is a lot better than my PC’s onboard audio).

Noise Isolation and Leakages

The 660S are open-back headphones which, while improving the soundstage, does leak quite a bit. They do not isolate sound at all, and while listening at low volume the fans on my computer would be audible, though not overpowering. It might prove distracting to other people in the same room, but when moving a couple of meters from the headphones on lower (but comfortable) levels the leakage is barely audible.

This does, however, leaves me kind of baffled as to why Sennheiser would market these to be used with mobile equipment: “Thanks to its low impedance it also offers reference-class fidelity with hi-res mobile players”. While possible to use with mobile equipment, the lack of noise isolation would make it highly impractical outside your home.

To drown out the hustle and bustle of the city, public transport, or an office you would have to listen to it at levels that would definitely annoy your company. You’re also likely to get tinnitus listening at those levels for longer periods of time, not something you want when buying premium headphones.


At almost $500, they might be a bit on the pricey side. For the sound quality, I still think they’re a strong contender in this price bracket. If they are ever on sale, I would recommend them to everyone who is looking for a pair of headphones that are great for acoustic tracks.


If you want to hear the details of the music, and you’re a fan of vocals, the Sennheiser HD 660 S are great. I heard a lot of new things that I haven’t before, and the instruments felt and sounded real.

If you need something bass-ier, or something that’s more musical, these might not be the right ones for you. The open-back format and the long cable makes the headphone more suitable for home use.

Bonus pic of my stereo setup: Tannoy Revolution DC6T SE, DLS Amp One, Rega Rp3, Rega Fono MM MkII, Hegel HD10 DAC (hidden).

The HD 660 S does a lot of things well but it does have its flaws. Ultimately, it comes down to your preferences. So if I’ve peaked your interest, you should definitely head down to the nearest store give it a listen!

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💬 Conversation: 4 comments

    1. So I sorta forgot about this review for a while; had too many other things in real life getting in the way, so I’m sorry about that.

      I would say the most important thing when choosing songs to listen to when testing headphones, is that they’re songs you like and know well. While being an audiophile I think it’s too easy getting caught up in the details of frequency respons curve, whether they are the smoothes, or have the lowest bass. It’s easy to forget that headphones are ment to be enjoyed. In a worst case senario you’ll be buying headphones that sound great on “recommended songs”, but when listening to music you enjoy personally they might not cut it. And you might also have a totally different perception of what is “shrilly”, “warm”, and etc. With that in mind there are no bad songs to use to test headphones, as long as you like the music and it’s something that you’ve listened to in some depth.

      Now, keeping that in mind I don’t really have a single list of music I go through when testing headphones. Rather I have some points that I want to check; the Bass, Midds, Highs, staging, separation, tempo, acoustic music, electronic music, rock, voice, drums, winds, reeds, classical etc. I usually end up choosing quite randomly among the music I know with these points in mind. I also try to try both high and low bit quality, and even some bad mixing.

      That said, there are some songs that I like that I think do a good job with these categories:

      Bass/electronic – Flying Lotus; “Tea Leaf Dancers”.
      A good tune as its use of sidechaining defines the whole track imo. It really need the low bass to sound it’s best.

      Midds/voice – Beady Belle; “The storm”. Ane Brun; “The Treehouse Song”,
      To me, the most important part of the midds is conveying vocals well. They should be warm and open, so I often end up choosing whatever a female singer I’m in the mood for.

      Highs – Coldplay; “Spies”.
      I don’t really listen to Coldplay a lot anymore, but I’ve always Chris Martin’s “s”‘s to be super sharp. If I can get through “Spies” without cringing from him hissing in to my ear, that’s usually a good sign.

      Staging – Pink Floyd; “Speak To Me”
      This is one place I would actually recommend going to one of the “certified audiophile playlists” as I don’t really have a lot of good recommendations. I usually end up going with the opening of “Speak To Me” for the staging.

      Separation – Hiatus Kaiyote; “Fingerprints”. Jacob Collier; “Don’t You Worry ‘Bout A Thing”
      As I said in my review, I used Hiatus Kaiyote to check the separation. I usually find vocal overdubs the hardest to separat, and on fingerprints there’s a lot of vocal overdubs. Jacob Collier is also a good suggestion. He’s firmly of the belief that “more is more”, so there’s a lot going on at the same time in his recordings.

      Tempo – Moonchild; “The truth”, D’angelo; “Sugah Daddy”
      Tempo is what convey’s the groove, and usually depends on the decay of the bass. With a slow decay you usually end up with the tune sounding “muddy”. D’angelo is some of the more groovy music I know, so if that dosen’t do it not much will. The same is true for Moonchild, which also doubles up as some nice milder electronic music.

      Acoustic (small band) – Keith Jarrett Trio; “My Foolish Heart” (Album).
      Brad Mehldau Trio; “The Art of the Trio” (Album)
      For smaler acoustic music I usually listen to jazz piano-trios. Being a pianist it’s important to me that the piano sounds right, and I also get to listen to the acoustic bass. I use Brad Mehldau for the studio recording, and Keith Jarret for live recordings.

      Acoustic (big ensembles) – Chick Corea; “Live in Molde” (Album),
      Trondheim Jazz Orchestra & Eirik Hegedal, W/special Guest Joshua Redman; “Triads And More” (Album)
      Helge Sunde; “Denada” (Album)
      London Symphony Orchestra/Prokofiev; “Romeo And Juliet” (Album)
      More jazz and some classical music. These ensembles have different compositions, and are some of my favorite albums, so I often end up going to these albums. The LSO version of “Romeo And Juliet” is where I get both the wind, reeds and brass.

      After these I end up going through my favorite artists, and trying to find what tunes that sound better/worse than my other headphones. Most headphones are good a different areas, so finding what yours excel at, and if you like that kind of music should in my opinion by the most important for deciding if you should buy them or not. You are after all buying headphones to listen to music, not get points from random people on the internett (for the most part).


  1. To be completely sure: Would you recommend upgrading to the HD660’s from the HE-400i’s? I have the HE-400i’s and even though I really like them, I cant help to seek out other headphones that sound better. The issue is that before I got into hifi stuff I had the Sennheiser GAME ONE’s which are basically the HD 598’s with a mic attached and branded for “gaming”. So when a friend of mine recommended me to purchase a DAC/AMP and the Hifiman’s I wasn’t really happy. The bass is definitively better, if memory serves me right; I think that some of the vocals was definitely missing, and it sounds like the hifiman’s are really forcing detail on your ears and they dont sound natural. YES the differences wasn’t really HUGE, but I’m a sucker for my mids and I LOVE vocals, and I would like to thing the HD 660’s would be better as the 598’s were better. Would you say that you agree with my statements? Not that the hifiman’s are terrible by any means, but they sound like they force details which affects the vocals, and most likely the mids in general, and the realism.

  2. It’s been some time since I’ve listened the HE-400i’s, and I havn’t listened to the 598’s so you should take what I say here with a heap of salt, as it probably wouldn’t be an accurate comparison.

    The thing is, reading other reviews of the HD 660’s the reception has been quite varied. Especially when they’re compared with the 600’s, and 650’s. Some people love them, some are quite ambivalent, and some thing they’re a clear step down from what made it’s predecessors great. So what does this tell us? Well, tast is subjective. Your taste in headphones might be different form mine, and that’s note even accounting for how are taste in music differs. That’s why you should try them out before deciding; go to your closest hifi store, and give them a couple of hours with your preference in music (preferably with a high bit rate). Or, if an online retailer has a return option you might want to buy form them so you can return them if you’re unhappy.

    Keeping that in mind, I would say that I agree with your assessment (from what I remember). While the HE-400i’s are great for the powerful rock/electro, and if you like a lot of sound, they sound quite closed of and unnatural to me. The HD 660S’s doesn’t have the same power to the bass as the Hifiman does, but are in turn more natural in its deliverance of the vocals and acoustic instruments. I think if you prefer more acoustic pieces of music, vocals, and don’t need the power the HE-400i’s provide you might be more happy with the HD660S. Now, if I understood you correctly your quite happy with the Hifiman, so I can’t say that getting the HD660 would be worth the money, or earth shattering in difference. Try them out, and if they’re close but not a perfect match, maybe try some of the other Sennheisers.


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