I have been listening to my HD800 for two days now, and my thoughts so far:
The bass quantity is more than sufficient, sometimes I feel like it is a tad too much
it is of a high quality,can go really deep but stays super tight..not a hint of flabbiness or mud.
The mids are fabulous! no need to expand on that..simply neutral. At first I was really annoyed by their known sibilance issue,but after some time with them and listening to different recordings,I realized that although the HD800 do emphsize the treble a little bit, the sibilance is really the recording's fault. The point is, that due to the HD800 little treble peak,they make it a little worse than it actually is. But when I listen to recording without any sibilance in them,the HD800 do not produce any sibilance all by themselves..just the opposite, they extend the highs beautifully. So one needs to avoid overly sibilanced recording with them, unless you want your ear pierced LOL.
I love them with all music genres I listen to,which is from classical to extreme metal ..electronic. Take into account that my gear is rather low/mid level. I use a laptop to a Dragonfly (1.2) which feeds a headroom ultra micro amp (the best part in my rig). I wonder how they will scale with better gear.
For conclusion: THE BEST headphones I heard to date! exciting just as Grado or Denon..or whatever..even more. People who say that they are bass shy or dry/boring sounding etc', simply don't like (or not used to) neutral sound,and prefer coloration (not a bad thing,I also come from this place). Imo, very FUN sounding cans.