Finally I had a short chance to listen to the HD820 from a HDV820, jazz and acoustic/pop played from Tidal.
It was much better than I anticipated based on the comments in this thread.
Indeed it is a bit honky, since a small bass range is weaker, but
- it sounded with much better bass that I expected
- the sound stage is phenomenal
- I liked it more than the HD800S and even the HD800
- no obvious treble problem
- instruments sounded rather nice (won't say life-like yet).
The HD660 sounds a bit similar, but the HD820 is much better IMHO.
Still, (on first impression) it sounded like a headphone that is not finished: there is a darkening, closing-in and honkiness that just should not be there.
Compared to the Denon D9200, the HD820 sounds much bigger, has a nicer and more dynamic view on music, but the D9200 is truly a reference-level closed headphone in what concerns extension, tonality, timbre (a first of its kind) and the HD820 is not yet there.
Congratulations
@Sennheiser for the HD820 work done so far, it has the chance to be in a different league than the rest of open and closed headphones, I was blown away by it, but pretty-please finish the job
. Alternatively please suggest amps that open the sound up, more so than the HDV820.