Not sure if this was in regards to my post or not but with limited HD 800 head time I will make a quick comparison just from memory.
Subject to change after more time with the headphones
HD 650 has slightly more bass, smoother sound, and less fatiguing in the treble. I'd say the HD 800 has good bass, more neutral sound, more transparent sound, treble can be fatiguing on certain songs ( Jackson 5 abc = very sharp and almost to a point where I can't listen to it on the 800's). Maybe it's the music I listen to or maybe my setup but I don't hear this incredible soundstage that everyone speaks of. It's def more spacious then the 650 but not by the near $900 cost difference.
Obviously these are opinions based upon a couple of hours of listening with both headphones, also with comfort issues with both headphones. HD 800 is very loose fitting almost to a point where the headphone doesn't feel secure and the 650 is too tight on the jaw (helped a lot with some stretching much more comfortable now)