I acknowledge my prior error. Flats are correct on the HP1. I seek forgiveness.
Clarity flows from many, many re-listens to toe's "Our Next Movement" -- which just has an absolutely ridiculous amount of deep bass, fast bass and funky bass all going on at once. Switching back and forth with HD800s, there is no slack or flop or murk in the HP1-with-flats presentation. The experience is different -- with the Grados shaking your brain around inside your skull while the Senns flap your auricles from the outside -- but to me they are both absolutely "right".
Supplemental observation. Anyone who says HD800s are seriously bass-shy is a heartard.