Just skimmed through arriving quite late to this thread - saw a few things mentioned there that I've got to hand though, so if anyone wants me to run any kind of test and report back let me know..

770, N810, Headroom Total Bithead, various Grado/Shure/Sennheiser headphones..

oh, and speaking of USB sound cards; Hercules DJ controller MkII - connecting that up might be fun and it's a generic MIDI controller so all sorts of potential there... would be awesome if something like the monome project could be set up with a Nokia tablet (even just with the touchscreen...)

Don't have a USB mini-A-to-micro-A cable though - I guess that's what you'd need to connect them?