Just received my HD 800 S.
I have recently verified that I value clarity and instrument separation above everything else, and I find this headphone to be astounding. Just like I wanted, I can focus on individual instruments even when it is super busy. I was considering getting the HiFiMAN Arya, but even those would be sacrificing too much on the clarity and instrument separation side of things for me, based on what I've read and heard on youtube. The headphone that made me fully realize that I am a "clarityhead" is the ZMF Verite Closed, which is way too congested for me, to the point of being completely unacceptable for a headphone anywhere near its price.
They aren't flawless, as they do lack a bit of impact to the sound, and they are on the thin side of neutral (as expected given the focii on clarity and instrument separation), but I was expecting both of these things.
Running them off a Massdrop THX AAA 789.