I also recommend looking into the Grado GS1000 and PS1000 if you are considering the Senn HD800.
I wouldn't go for GS1000's - I've had two different pairs. Soundstage they have in spades. Great bass? Not really, but OK. They also have a slightly recessed midrange which seems to work well for classical, but not as well for anything really mainstream (rock, pop, vocals, etc.). They are not like anything else in the Grado line, and they are rather distinctive headphones. They are completely not like the RS1's you are considering. They are outstanding cans for low-level listening (probably the best I've heard). I actually like them, but I much prefer the more versatile HD800's. For both bass and sounstage in the Grado line listen to the PS1000's. I'd try to buy your amp according to he heaphones you get and the music you like and how you like to listen to it. Headfi.org is a good resource for all things headphones if you can wade through the noise. No experience with the Headroom amp to make a comment.