The upper midrange/treble troubles me too, but I don't hear it as a nasal inflection. Rather as a sometimes-intrusive sharp edge to vocals. I do absolutely agree, though, that the "sweet spot" volume level is too low for my liking.
Where I part company with the sloths and join the elephants is on the spaciousness of the presentation. Elephas is spot on about the sound staging. I just couldn't go back to the 6X0 series now, despite having lived happily with both for extended periods in the past.
The HD800 is a superb headphone. But like everything else in life, it has its own particular frustrating imperfections.