I say it's a tossup between the HD800. Really no surprises on the comparison though... The Senn has better staging and imaging and overall microdetail, while i feel the HE-6 has better midrange presence and much more body in the low end.
The HD800 is pickier with amping though, moreso than the HE-6. The HD800 can change character depending on the amp used; the HE-6 scales up but more or less retains it's character.
This makes sense based on the disgusting impedance graph. I'm thinking that monster needs an amp that compensates for the nasty impedance hump. It's not flat like HE-6.