I got my HD800S yesterday and have put around 5-6 hours of listening on them so far. For context, my system is NAD M51 > HDVA600 > headphones, listening to a mix of source files from my Windows 10 machine in either 256-320 kbps compressed formats or lossless 16/44.1.
My initial impressions are very positive, but having owned and enjoyed the HD800 over the past few months (and on a couple other occasions a few years ago), this didn't really come as much of a surprise. I agree with most of what has been said about the HD800S versus the HD800. The HD800S does have a tamer response around that infamous 6kHz mark, but I think it still is very much a bright, airy headphone. I would say that it has a neutral sound signature, but I still think it rides on the brighter side of neutral when compared to, say, the HD600, which sets the benchmark for mid/treble neutrality for me. I haven't really noticed any meaningful difference in the bass response between the HD800 and HD800S. Both of them provide a pleasing amount of bass quantity for my tastes, and the bass is tight, impactful, and very well extended.
So what are the implications of that reduction in the 6kHz peak? I think there are both positives and negatives. At first blush, I find the HD800 to be the more immediately engaging headphone, exhibiting a slightly larger soundstage with a touch more air and slightly sharper imaging. It also provides a sense of energy to the attack of percussion strikes and string plucks that is ever-so-slightly diminished on the HD800S. However, I also have to say that I found the HD800S less fatiguing, even though I never thought the HD800 too fatiguing. This is noticeable on rock recordings, where the HD800S handles electric guitar in a gentler fashion. It was especially obvious on Aerosmith's "Janie's Got a Gun," where Steven Tyler's vocal harmonics can be piercing on the HD800 but are better tamed on the HD800S.
Obviously, it all comes down to personal preference, YMMV, etc. The best way I can sum it up is that I found the HD800 to have that immediate "wow" factor that makes them special, whereas the HD800S was a bit more even-handed and able to handle a wider variety of source material with grace. I like both headphones very much, but I don't think they are different enough to warrant keeping both -- and because these are going to be my primary headphones, I think I will be keeping the HD800S for its more forgiving nature.
On another couple of points -- I like the black finish on the HD800S; hopefully it will hold up better than the silver on the HD800, or at the very least scratches / chips should be much less visible. I always liked the stock HD800 cable, and the balanced cable that comes with the HD800S is much the same. It's a medium-weight, fabric-wrapped cable that seems durable and has good ergonomics. I couldn't comment on the sound quality of the cable versus aftermarket options -- I've never been a big believer in sonic differences between cables and I'm really only interested in balanced insofar as trying to squeeze the maximum possible performance from my DAC and amplifier.
