Maybe Sennheiser would suggest that you can drive this headphone directly out of a smartphone, but I consider it an April’s Fools Day joke. This headphone need the cleanest possible amplification and, of course, a dedicated one. I’ve managed excellent results with an Audio GD Master 9, and also with a HeadAmp Pico Power, and even with the ordinary Pico. The headphone simply comes alive, the dynamics raises your pulse, it’s a completely different story with dedicated amplification. Everything else is left at default, the master is usually brought at least at -7dB to avoid clipping. When changing the HD 650 with the HD 660S after a non-stop 2 hour listening to the HD 650, I can feel that the veil has lifted, the mid-bass stops rattling and clipping and the harmonic distortion feels lower.

In conclusion, I don’t recommend you to plug them directly into a smartphone, but if you add a portable amplifier to the mix, things will get radically better. If you listen to acoustic music, jazz or classical, the HD 660 S becomes an obvious recommendation. My final thoughts on the HD660 S is that I believe that Sennheiser is making a mistake in calling it a successor to the HD650, for to me it’s not. The HD660 S is a great headphone but it’s its own personality and presents audio in its own way that I believe is way too different for it to be considered a successor. The HD800 S, to me, is the definition of what a successor is. But as an individual headphone the HD660 S is wonderful and a real treat to listen to. I do like how it can be played more friendly off mobile devices however I will say that in doing this Sennheiser has really handicapped the HD660S’ ability to scale. It still shows improvement when listening to top tier gear over a mobile phone but NOWHERE near the legendary degree that the HD650 can. The lower-midrange of the HD660S sets it quite a bit apart from the HD650, to my ears – but more on that later. It is decently represented without slipping into the overly warm category. Make no mistake, I’d still classify this as a bit of a warm headphone (all things considered) but it certainly has less emphasis in the region than I expected. I found male vocals to do really quite well, and found the chugging effect of distorted electric guitars to have ample body and weight to them. Now That We’re Dead by Metallica is a good example of both these factors in play, with the bite of the rhythm guitars being in full effect.

The bass is a different story. The HD 600 has a weaker bass and the HD 660 S has a deeper and faster bass. The bass is the only area where the harmonic distortion can become a problem, especially if you tweak the EQ too much. The HD 660 S scales much better in this regard, offering a lowered harmonic distortion in this area. However, if you’re after a more neutral sound, you should also consider the Shure SRH1840, HiFiMAN HE560 or an AKG studio headphone such as the K712 Pro. The basic signature can be described as slightly dark compared to something fully neutral, but not overly so. It is warmer and more musical than something like HD600, it has more detail, more clarity, and it has better definition than HD650. It would be more of a headphone to satisfy both the users who love HD600 and those who love HD650, bringing forward the best of both worlds. In any case, if you like a slower smoother presentation, I think you’d better keep the HD 650 because it does this better.

The highs on the HD 660S are definitely the brightest of the HD 600-series headphones, and while I would not call them veiled, they are definitely still on the warmer side. For frequency response I only had two minor issues with the HD 660S’s treble. The more noticeable one was a peak in the lower treble at 5.5k, that–like on the HD 58X–made the HD 660S’ timbre sound a little bit more congested than on the HD 6XX and HD 600. Additionally, the frequencies above 10k could use a little more energy for my preference, I thought that they sounded a little muted. Aside from that, I still really enjoyed the treble and I found it to be very smooth overall. For resolution I think that they have very good detail in the highs; they are very resolving and clean in their delivery. Comparing them to other Sennheiser headphones, I thought that for resolution they were a little more resolving than the HD 6XX, and were a significant upgrade in treble detail from the HD 58X. I personally thought they were more detailed in the highs than the DT 1990 Pro, and they were on par with the HiFiMan Sundara. Soundstage and Imaging As long as you’re buying for the home rather than portable use, there’s very little to dislike about the Sennheiser HD 660 S design. While we don't know Sennheiser's input in Equalizing their headphones, we always provide those were we're able to. The main thing that we felt would be welcome to add to Sennheiser HD660S was a touch more bass (and a lot more bass for certain music types), along with more treble. Happily, the Sennheiser driver technology is quite good and it can be equalized quite a bit without losing definition and without introducing distortion. But let me now talk about the individual characteristics of the sound signature so that I may be able to convey my thoughts on the HD660 S better. So I really like the mids on the HD660 S and I think when considering the this headphone its own headphone, I believe most will as well.The mids on Sennheiser headphones are a complex thing to describe. Straight out of the box they’re a bit stiff and muted. But as time goes on and you get a proper burn-in time I find the Sennheiser sound signature is one of my favorites… especially in the mid-range. This is also true of the HD 660 S. It’s a little muted at first but in time it really develops a pleasingly crisp mid-range response that compliments nearly all music genres. Highs These days, portable headphones tend to be at the top of most buyers’ to-buy lists. However, the Sennheiser HD 660 S prove full-size open headphones such as this are generally a better choice if you simply want a pair to use at home. The bass of the HD600 is the most muted of the three headphones and it remains largely in its own camp and doesn’t intrude like the HD650 can at times. There is still a heft in the low end of the headphone that doesn’t completely eliminate the sensation of bass, but it’s hardly ideal for genres dependent on slam and rumble – for which the HD660S is a better choice. Overall impact is also less frantic than the HD660S, but seems slightly more pronounced than the HD650. The sub-bass rumble in Why So Serious? is also weaker and less controlled than the HD660S, but the instrumental has a certain sheen around it that is interesting. It’s not smoothed, like the HD650, but it’s definitely a softer and less harsh experience than the HD660S during the crashing moments. Sennheiser would have you believe that you can just go and run the HD 660 S straight from a smartphone. While it’s twice as easy to power this headphone it’s a bit of a stretch to use a smartphone directly. I tried it and the sound did play out but I found myself pushing the volume all the way up and needed more. So do yourself a favor and always use an amplifier with the HD 660 S. Final Analysis

Buy the Sennheiser HD 660S at headphones.com at the best price, and its the best return-policy available. The comfortable soundstage and detailed imaging hasn’t changed much. This was kinda difficult to determine because the forwardness of the mids and upper bass makes the soundstage sound a bit closer than the original HD650 did but when playing normal classical/instrumental music, at least me personally, I can’t hear a notable difference between the two in the soundstage and detail side of things. Schiit Wyrd > USB Version 5 Schiit Yggdrasil > Dragon Inspire IHA-1 > Sylvania Bad Boy 6SN7 + Philips Metal Base GZ34) Maybe if you like a more lazy, relaxed and less dynamic sound with less detail, then yes, the HD 650 can bring exactly this to the table.Of course, this may not be best for all users since weight can add to the feeling of rigidness and reliability of a product, but still, it is impressive that Sennheiser managed to pull this off. It makes a refreshing change to see the launch of a set of headphones that are focused entirely on sound quality, rather than the latest flashy wireless standards and features. Yes, they are still here, they haven’t left. Probably the biggest virtue of these headphones is the very present mids that seem to infuse joy directly into your central nervous system. Talking about the price, the HD 660s being a new model, at the beginning of the life cycle, 500 EUR seems to be a reasonable price for what they are. The HD 650 had the same release price. At the moment the HD 650 can be bought for much less than the official original price, but let’s not forget that they’re 14 years old, it’s just normal to be much cheaper now. Like the build quality, comfort is also very good on the HD 660S. Thanks to their mostly-plastic construction, these are extremely light (only 260g or so), so I doubt anyone will have weight issues with these. The pads use a fairly dense foam wrapped in velour and are very soft on the skin, I only wish that they were a bit wider on the inside as some people’s ears might touch the inner sides of the pads. The only problem I have with the comfort is that out-of-the-box, the clamp force can be quite vicious. Nonetheless, this can easily be alleviated by extending the headband all the way out and gently flexing the part that extends.

The uild quality includes mostly plastic, Sennheiser HD660S has some other materials that are not plastic, like for example the drivers, or the grills on the drivers / the padding, but this shouldn't be taken negatively, since there are plastics that can resist to extreme amounts of weight and pressure. I observed similarities in the dampening material behind the diaphragm and in the metal mesh behind the diaphragm. Its shape also resembles that of the HD 700. First, we need to consider the value HD660S has at the moment of writing this review. They sell for around 500 USD in most parts of the world, which places them in direct competition with pretty amazing headphones like Beyerdynamic Amiron / DT1990Pro and Ultrasone Signature DXP / Signature Studio.I'll not wax poetic this time regarding the long and storied history and my experiences with the HD 6xx family of Sennheiser headphones. For that, simply go have a look at my " The Very Important Sennheiser HD 580, HD 600, and HD 650" review.