I've really been enjoying the HD660S2 for the past 2 weeks, waited until now to get a short review together.
I have been comparing the HD660S2 to the HD600, HD660S (OG), and Focal Clear MG.
First thing I have to say, the bass is a great improvement over the original 660S, and I can safely say this is the best and cleanest bass I have heard in an HD6XX series headphone. To me, the bass just sounds straight up better than any of the other 6XX headphones. It sounds noticeably tighter and more defined than all of them, and I guess that's not surprising considering the improved sub bass was Sennheiser's marketing for this. I am also hearing better dynamics and just an overall more controlled sound over especially the 600 and 650. The 660S is very similar in this regard, but the lack of sub bass extension on the original 660S is noticeable in direct comparison. I definitely prefer the new S2. The S2 reminds me a little of the Focal headphones in terms of dynamics, although it's definitely a step or two down from that level, great but not top tier. I'd say it gets close though, the 660S2 is still a very snappy and fast sounding headphone, despite it having a warmer tonality. I've just been spoiled by the Focal Clear MG, which is more dynamic and punchy than any other headphone I've heard (including the Utopia 2022). The S2 has a better sense of punch and attack and speed than its siblings in the 600 lineup. The bass is cleaner on the Clear MG, unsurprisingly. I don't have the Elex anymore, but I think that would be a more interesting comparison because of the similar price ($599 vs $700). I can say for sure though, the midrange is more "liquid" and smooth on the S2 than any Focal I've heard, and the midrange also comes through clearer with more presence than the Focals.
Speaking of the midrange, that is one thing every Sennheiser 6XX headphone has always done better than most of the competition. The midrange is just gorgeous on the S2, the timbre is very natural and organic. Instruments and vocals especially are Sennheiser's strong point, and the 660S2 is no exception. While the S2 is somewhat warmer than both the 600 and original 660S, the detail sounds better to me. Aside from good detail, surprisingly I'm hearing more refinement and naturalness in the treble vs my Focal Clear MG and Hifiman HE-500 (not necessarily more detailed though). The treble also sounds like an upgrade over the other 600 series headphones, as good as they are. Another advantage the S2 has over the open back Focal headphones, the midrange and treble just sounds more cohesive and linear. The Clear MG has more sharpness and "glassy" timbre to the treble, which isn't all that natural much of the time, at least compared to the Sennheiser headphones. I'm aware I'm comparing a $600 headphone to an over $1,000 headphone, but it really goes to show you how well the 600 series have held up against the modern and more expensive stuff coming out. I would say the same about the older 650 and 600 which are around $300 now, but the S2 takes the resolution and technical performance up a notch. The performance of the S2 is very good, but don't expect it to beat the Clear MG in the areas of dynamic punch, soundstage width/depth, and bass resolution. However, I think the imaging might be taller on the 660S2, while the image is deeper on the Clear MG.
I prefer the midrange tuning of the S2, as well as the original 660S, over the 650 and 600. The 600 and 650 are excellent, but they always sounded a bit too forward for me. And although I did get used to it over the years, I generally prefer this more neutral tuning of the upper mids/lower treble. The only complaint I have about the tuning on the S2 and the original 660S (specifically this upper mids/lower treble) is there is slightly less presence at 3-4 kHz than I would like, some instruments lose presence and sound a bit dark sometimes. This however can be fixed easily with EQ, and even then I only add 2 dB in that frequency area. The S2 makes up for this slight recession by being richer and cleaner in the midrange than the older 600 models. The S2 also adds some space to the recording as opposed to the HD600 and HD650 presenting the music very close right directly in front of you, sometimes to a fault (granted this does work well with some recordings). Much of my impressions versus the HD600 line up exactly with what
@SLC1966 said in their comparison on here. The 600 is very close and in your face while the 660S2 is better at surrounding and immersing you with more richness to the music. With the HD600 vocals being too forward for my tastes, and the Clear MG being a little too pushed far back for me, the HD660S2 is just perfect right in the middle of the two and now my favorite midrange/vocals presentation. Vocals and midrange centric instruments come through more naturally on the S2 compared to the upfront nature of the 600, and somehow the S2 sounds clearer and cleaner despite being warmer. But in particular, I think the S2 really shines with male and alto female voices. I recently modded a 6XX with the Custom Cans copper mass load and ZMF Suede pads, and as good as that sounds, I still think the 660S2 by itself with no mods sounds better. Also, the S2 really takes EQ very well, especially in the sub bass, much better than the 600 or 650 could ever do. You can hear it extend all the way down to 25 Hz using a sine sweep, even without an EQ boost. I would like to say something controversial many reviewers won't agree with, that the S2 is the HD600 + HD650 sub bass upgrade we have been waiting for, it outclasses the older models in that area.
All in all final thoughts, I'm really glad I picked these up. These are everything I wanted the original 660S to be. It still has the signature Sennheiser HD6XX series sound, but with even more welcome changes vs the regular 660S. It's basically the perfect Sennheiser headphone for my tastes. The sound presentation is warm and very natural, yet without the loss of intensity and engagement. The HD600 still has it's place in my collection, but for an all rounder that works well with all genres, I'd pick the S2. I'd go so far to say I like them better than the HD800S (from what I remember), not in terms of soundstage and imaging but as a whole package. Then again, I prefer a more neutral/warm tuning than the HD800 series. With a little EQ in the bass and upper midrange, the 660S2 is quickly becoming my new reference headphone.
Also, I'd like to share that this track below from the game Ori and the Blind Forest on the HD660S2 actually made me tear up. The female vocals come through so beautifully on the 660S2. Sennheiser is still king of vocals. And Mahler sounds pretty awesome too.
