Review: Sennheiser HD660 S – Blended

Sennheiser HD660 S

Bass, Mids, Treble

 

I’ve seen a couple of people describe the Sennheiser HD660 S as having only a very light bass, and to me that shows they definitely didn’t try the HD660 S with different sources and/or even the balanced cable. Bass, depending on the amplifier and connection used, goes from neutral and fast to bigger bodied and more romantic. Some might not like the bass dependency but I actually think it’s a good thing as you can make the HD660 S’ bass sound like you want it to sound. In both cases bass has great detail, nice layering and good depth.

Sennheiser HD660 S

Sennheiser HD660 S

It’s more or less the same story when it comes to the mids. They go from fast and precise with a little lighter body, to a thicker mid presentation that’s a bit slower yet smoother and very rich. The mids in both cases have excellent timbre and detail and just like with the HD650, the mids are oh so important in the HD660 S. You either like the typical Senn mids or you don’t but to me they’re musical, rich and well layered with a natural tonality.

The treble section in single ended mode – and from more neutral sources – sounds energetic, neutral and fast. Some might even find treble to the brighter and more forward side but treble is nicely extended, clear and crispy. With warmer sources and in balanced mode, you get a softer and maybe more natural sounding treble, still with good energy and lots of detail. Of all 3 Senn headphones, the treble on the HD660 S might be the most particular, something that’s noticeable in the FR chart as well.

If you’ve been reading up about the HD660 S on the web you might have come across some statements about the HD660 S graininess. While I do see what they’re pointing at – especially when coming from the MX-4 – I can’t really say it bothers me this much. Is the HD660 S the “leanest” of the threesome and the most difficult to get right? You may have a point there.

Sources / Amplification

An important point with the HD660 S is that it is easier to drive than the other, 300Ohm, Sennheisers. While this is a fact, you will still see the volume going up on your source when coming from a really easy to drive headphone. And it doesn’t stop there as the sound signature from the HD660 S really depends on the source you’re using it with. Like its brothers, the HD660 S scales up nicely with a good and powerful desktop sized amp, though it also sounds more than just good when connected straight to a DAP. In that regard the new HD660 S is a little less “upscaling”. I would still suggest using a dedicated amplifier however as it just lifts up the HD660 S’s performance, and basically Sennheiser is saying the same thing:

Offering a lower impedance of 150 ohms, the HD 660 S can be enjoyed with HiRes mobile players or when directly connected to high-quality, stationary HiFi components. They fully reveal their potential when connected to the balanced outputs of a dedicated headphone amplifier such as the Sennheiser HDV 820 or to the balanced output of a mobile digital audio player.

Sennheiser HD660 S

Sennheiser HD660 S

Desktop Sources

Ever since the Headonia and Solaris were added to my main headphone system, the Violectric V281 doesn’t get as much play-time as it used to. The Vio V281 is the only solid state amp left in my setup and I really should listen to it a lot more, as it is an exceptional performer. Both the older HD6-series sound great in combination with Violectric amp(s) and so I had really high hopes for the HD660 S as well. The V281 didn’t disappoint and you get a full bodied, still nicely paced bass with good depth. The mids also have good body and the treble section is a tad softer as you’d expect from the V281. The only point where the V281 in single ended disappoints a little in combination with the HD660 S, is with the spaciousness and sound stage. If you’re familiar with the amplifier however, this shouldn’t come as a surprise.

With the Astell&Kern Acro L1000 you get a fast, neutral presentation with somewhat lighter bass, but with excellent speed and precision. Treble for some might be a little too spoken out but the mids are still lovely detailed and precise. It’s not a great source for the HD660 S but it certainly isn’t a bad source either.

When switching to the more neutral tuned Auris Audio Headonia 2A3 tube amp, you get an overall more energetic sound with a higher pace and more forward upper mids and treble. You also get deeper/tighter bass, more neutral and fast paced mids with slightly more forward voices and more top end extension and clarity. Treble becomes more energetic but I’d not label them as bright per say.

Sennheiser HD660 S

Sennheiser HD660 S

The warmest and smoothest HD660 S shows up when paired to the AudioValve Solaris tube amp. It’s a softer and romantic sounding amplifier and the HD600 S immediately shows that. You get a smooth and rich sound with more impactful bass (great layering!), thicker/warmer musical and natural mids and softer yet extended and rich treble. The musicality here is high and the HD650 fans will for sure prefer this combination where the HD600 fans will probably prefer the Headonia as source. The V281 is the perfect amp for those who want it to sound a bit like both.

Portable Sources

The Astell & Kern AK70ii is a complete player with a warmer sound but one where the musicality level is very high. It especially are the vocals that jump out in this combination and you get a good amount of bass, a good pace and soft treble but the mids aren’t the thickest overall. While this combination certainly isn’t bad, it doesn’t really “do” it for me either as bass is undefined and energy is missing overall.

The TOTL Astel&Kern SP1000 is a more neutral tuned player, with a very high level of detail and a focus on the upper mids and treble region. It has a fast pace, tight bass and great clarity and with the HD660 S hooked up to it, the combo sounds really good. Tight, powerful bass. Excellent clarity and speed. Rich, clean mids. A wide sound stage, etc. If you prefer warmth and smoothness (or the typical HD650 sound), you might find the sound a little too energetic and forward however. In that case the NW-WM1Z is more up your alley.

With the Sony DAP, even in single ended mode, you get a warmer, slower sound which also is very musical. You get bigger bass, thicker mids and soft yet detailed treble. The overall level of detail is high and the Sony/Senn combo shows great layering. Switch to balanced mode and the bass becomes impressive, the mids even smoother and the treble softer. This is the ideal DAP if you’re looking for that HD650 sound signature. The SP1000 is more comparable to the HD600 for that matter.

Sennheiser HD660 S

Sennheiser HD660 S

With Chord Electronics Mojo (AK70ii as source), you get a very fast sounding HD660 S with great detail and extension. The focus here is more on the upper mids and treble as well (like with the SP1000), but it’s softer and smoother in presentation with great vocals. Bass is lighter in body and impact but it is good in quality. Treble is energetic but not the furthest extended. With Chords Hugo 2, you get a higher resolution and more extension with improved depth and layering. The pace is still fast and you get a clean, more to the neutral kind of sound, yet with a slightly smooth touch to it. Bass has more quality with better depth and layering. This also is valid for the mids yet the upper mids and vocals still have that focus on them. The treble end is more extended compared to the Mojo and it has more energy, and might come of brighter to some. Personally I prefer the Chord Hugo 2 with the HD660 S as it lifts the Senn up to a higher level.

I have to say I was positively surprised with how musical the HD660 S sounded straight from my laptop. Sure bass impact and body was on the lighter side, but it was tight and punchy. The mids were rich and the treble energetic. The voices were more up to the front but not in a way that it becomes annoying. It’s an excellent emergency setup, let’s put it like that.

In the end it is clear that the HD660 S can indeed be reasonably easy driven by a portable source, but I still prefer a full sized desktop amplifier. My personal preference goes to the Solaris, while in portable mode, the WM1Z in balanced mode holds my preference.

Sennheiser HD660 S

Sennheiser HD660 S

Conclusion

Is the Sennheiser HD660 S a good headphone? Absolutely! Is it a great headphone? Yes, with the proper source it definitely is. Is it as good, if not better than the HD600 and HD650? Well, there it becomes more difficult. The HD660 S is a great headphone if you want a bit of both the HD600 and HD650, but in my personal opinion it won’t ever be as iconic as the HD600 and HD650 already are. In 10 years’ time people will still be talking about those two Senns, but I’m not sure they’ll remember the HD660 S. Only time will tell if I’m right or wrong.

It is a fact that the HD660 S now is easier to drive, and it performs really great with the Sony WM1Z in balanced mode, but the headphone still scales up nicely with a good desktop sized amp, yet not as much as before.

I have been using the HD660 S for about 3 weeks non-stop now in my office and while I liked it from the beginning, I like them even more now. So yeah, the HD660 S is a job well done and it will please a new group of listeners that couldn’t find their ideal sound signature in either the HD600 and HD650. I don’t think that those who love their HD600 and HD650, will be replacing their beloved headphone for the HD660 S, but if you have to start from zero, the HD660 S might just be that one for you with a bit of the best of both worlds.

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Lieven is living in Europe and he's the leader of the gang. He's running Headfonia as a side project next to his full time day job in Digital Marketing & Consultancy. He's a big fan of tube amps and custom inear monitors and has published hundreds of product reviews over the years.

23 Comments

  • Reply April 3, 2018

    Musikverein

    A sane and in a word balanced review. Kudos, Lieven.
    The best thing of the 660s to my mind is, besides of the psychoacoustic blended-ness, are its convenience factors so to say.
    First, it has all necessary cabling coming with it and you will just need an additional adapter f.i. 2,5 to 4,4mm balanced and then that is it. You arrived a high ends heavens gate.
    Second, it can be driven with stuff starting with a Dragonfly Red, iFi nano, Mojo etc. to certain satisfaction of even audiophile gourmets. This indeed is new in HD6.. land when until now you needed serious solid state or tube based gear, to be honest 🙂

    • Reply April 3, 2018

      dale thorn

      Great observations, you can’t lose with the 660s.

    • Reply April 4, 2018

      Lieven

      Thank you. That is definitely the case and I applaud the multi-cable thing 🙂

    • Reply May 30, 2018

      Veri

      Indeed. 6-series sound without breaking the bank (too much).

  • Reply April 4, 2018

    Michel I

    Nice review again, Lieven, thank you.

    However, I don’t share your conclusion. The differences are small with the older HD600 and HD650, and there is a significant difference in street price: at this moment in Europe 429 Euro for the 660s but only 262 Euro for the 600, 315 Euro for the 650, and 435 Euro for the 700 (amazon.de).
    The balanced cable is exotic; most of us will need an adaptor or another cable to use it with our balanced headamps.
    The 600 is the best choice at this moment !

    • Reply April 5, 2018

      oliwek

      … then I still wonder : is the 600 the best choice when paired with portable sources, such as Mojo (single ended) and above all balanced with something like Hiby R6 (with a 2.5mm TRRS to 4.4 adapter) ?
      (if some people can comment on the results they get with HD600 or 650 with Mojo, you are welcome, as the reply from Musikverein lets me believe it’s maybe not ideal).

      • Reply April 5, 2018

        Michel I

        I think you need a good, powerfull amp for all the members of the 300 Ohm family, including the Massdrop x Sennheiser HD 6XX that you can buy this month on massdrop.com at 200 $ ( + 15 $ delivery outside the US), but also for the 150 Ohm 660s. The power of a Mojo is simply not enough.

  • Reply April 5, 2018

    Hans

    Hi Lieven, great review!

    The review grabbed my attention when you mentioned: “With the Sony DAP, even in single ended mode, you get a warmer, slower sound which also is very musical. You get bigger bass, thicker mids and soft yet detailed treble”

    Although this is the NW-WM1Z, the bigger bass, thicker mids and soft detailed treble was exactly what I thought when I heard the NW-A45 compared to LG V20.

    Maybe one day Headfonia can review the NW-A45 and see if the implementation of the same digital amp stays faithful to the Sony sound signature for much cheaper 🙂

  • Reply April 5, 2018

    MhtLion

    Thanks for another great review!! I have V281 and Senn HD6XX, which I love the paring but just sometimes I wish for more treble energy. I was torn between HD600 vs HD660S while I’m waiting for HD820 to be released. I have a sense that out of the balanced of V281, HD600 maybe the best out of three for my taste.

  • Reply April 6, 2018

    Marc

    Recently treated myself to HD 660S to replace my aging HD 580 and paired it with the Massdrop Cavalli CTH + SDAC combo. Sounds great!

  • Reply April 18, 2018

    Dom Sweat

    Although I have yet to try a pair of open-back Sennheiser ‘phones I’ve heard nothing but good things about them.

    The Sennheiser HD 660s seem like a pretty good option for consuming the more spacious sound that these are well known for.

    Would these be a good entry level pair of open back headphones or would you suggest trying a less expensive pair at first?

    • Reply April 18, 2018

      dale thorn

      I wouldn’t go less expensive than entry level. The 660s is a great headphone, and how much you’ll like it will depend on many variables – your music, your source, your amp, etc.

  • Reply May 19, 2018

    Lukas

    its good headphones for games? which is better with better sound stage? Hd 660 s or Hd 700 ?

  • Reply June 22, 2018

    DC

    Lieven, for those looking for the smooth and slow sound with pronounced bass, but worth going for the 650 or would it be better to go for the new 660s and try a smooth signature amplifier to make the headphones more romantic?
    You say that with the right setting the 660s gets softer and with more pronounced bass, looking like the 650, so this 660s soft signature is nicer than the smooth natural of the 650?
    better go for the 650 that is smooth by nature or try to leave the 660s romantic using an amplifier that leaves its signature?

    • Reply June 24, 2018

      Lieven

      In that case I would still go for the HD650, as it does that best

      • Reply June 26, 2018

        DC

        Thank you Lieven! If the 650 is the only one with this smooth signature of the HD6xx line up to the 800s, sennheiser should not discontinue the 650. I almost sold my 650 recently to buy the 660s hoping for a smooth signature but easier to amplify. But the King is still king, I will continue with the 650, because it was the right choice of signature that I made, with the help of serious reviews like yours.
        The sennheiser 650 has been improving over the last 14 years, including the easiest to drive drivers and more compatible with multiple amplifiers, they also said the newer paint is more resilient.

  • Reply September 20, 2018

    Matt

    Hi Leiven, thanks for this review. I’m considering getting some over ear headphones for when I travel overseas to replace my Shure SE425s. I listened mostly to classical music. Would the HD660 be a reasonable match with a Fit X5 2nd Gen DAP?
    Thanks for your help

  • Reply September 28, 2018

    Juan

    Awesome review… Love Headfonia reviews. Any chance we will get a HD820 Review anytime soon?

  • Reply September 28, 2018

    Lieven

    Not sure when but yessssss

  • Reply October 9, 2018

    Oost

    Thank you! Very informative. Any comparison to the LCD-2(C/F)? Both with Mojo as amp.
    Which is best for classical music?Piano, vocals, symphonies?

  • Reply November 29, 2018

    Moosa

    Thanks a bunch for this detailed review
    Do you think the KANN can be paired with this headphone?
    Thanks

    • Reply November 30, 2018

      Lieven

      Oh yes, though a bigger amp is always better

      • Reply December 1, 2018

        Moosa

        Thank you so much
        So this means I will definitely get it:)
        Thanks again

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