HD820 is much fuller sounding compared to HD800S. To my ears, bass response is also better than HD800S. Bass extension is probably the same but with slightly more quantity and stronger slam.So it's just an HD800s that's closed back? No increase in bass response or anything, just the same sound sig but closed?
Sound stage is wide for closed cans but clearly narrower than HD800S, which is good for me because I want to be "in" the music not outside.
The only problem is that the clamp force is a bit loose. HPs stay on the head ok. But when I press the housing against my face, bass quantity dramatically increases. I prefer a little tighter fit that would provide even more bass. Don't get me wrong, the bass quantity without pressing the housing to my face is just good enough and very well balanced. It is my personal preference to have a bit heavier presentation some times.
If you love HD800/S and looking for an upgrade, look elsewhere. If you cannot stand thin sound signature of HD800/S like me and are looking for quality closed HPs, I highly recommend HD820.