I really hope the HD820 starts a revolution among closed headphones, like the HD800 did in its own time among open headphones, so other makers will also pull their facts together (particularly I am thinking about the AKG K872 fiasco). It is nice to hear that Sennheiser has been there and abandoned that approach.
I am not fond of glass back, not sure if e.g. carbon fiber would not sound better, but the visual appearance with glass is very original and looks fantastic - in the shop. When on my head, I just care about the sound. And even in the shop, I care more about the price than the exhibition.
If it's near-neutral with a slight sub-bass boost (as heralded) over the HD800S, with slightly less wide sound stage than the HD800(S), then that sounds like
perfect to me. The harmonic structure of the timbre, low distortion, attack, decay, low level resolution matters much more in bass perception than bass amplitude. For instance, I have an e-stat (pictured in my avatar) that sounds classes better in the bass than my Fostex, Denon and Sony closed cans, even deeper, fuller and more impactful, while measuring near flat in the FR and while being much more clear. Too bad it's open and sounds like a speaker, actually needing
dedicated and possibly
sound-insulated listening room in order to avoid sound leakage
from one room to another... so I completely get Axel's jest about Europeans
. So don't think the HD820 is mainly for Asian markets
.