DivineCurrent
500+ Head-Fier
Nice impressions of the HD800S vs HD650 -- they match mine pretty much exactly! I think the HD650 is more linear through the upper mids / lower treble, whereas the HD800S (and HD800, for that matter) has a slight, broad dip in that region. I'm kind of sensitive to that frequency range, so I actually find the HD800S slightly less fatiguing than the HD650 (although it seems most people would probably say the other way around).
Lol, believe it or not, I find the 650s more fatiguing too! At least in that frequency area. I think it is directly a result of the more intimate nature of the 650s. You actually get more detail than the 800S in the high mids because of how close the drivers are to your ears. But with the 800S, that dip is probably due to the greater distance the drivers are from your ears. Makes for a very speaker-like experience, which I always enjoy.