I had recent meetings with both. I haven't used them for a while. I also found out a bit more about HD820 and HD800S.
My wife bought a 3 CD pack of Tina Turner. I hadn't listened to her music for a while and was excited.
I started with HD820, but it didn't come out well. Highs were too piercing; Tina's voice is that kind of voice...it's hiiiiigh, but at the same time, it's gooood.
But no, it just didn't work; I had to lower the volume so I could stand the voice, but then the whole energy came down...
Tina and her music are pure energy; you can't do that, it's an insult!!
Then I took HD800S, and I didn't expect what I got. I thought if HD820 are sharp with highs, then HD800S would be even worse.
But what happened was the opposite! While HD820 sounded like a V-shape, HD800S sound was nicely balanced and mellow. Tina's voice, as it was high, suddenly got some silkiness. The pleasure of listening came again...it was great! HD820 with Tina was almost lifeless, and with HD800S was full of life and body!
The next day, I used HD820 and listened to Drum&Bass, and HD820 were again so good!
Until now, I thought that HD820 were an all-rounder, but they are not. The best way is to use both; they are so good as a pair.
Cheers!