Depending on your budget, I have a Darkvoice 337 for sale which is said to be a wonderful amp, and I got it based on Skylab's review, and it actually is the exact one he reviewed! I am not just saying this to try to get it sold.
These are others opinions though, as I have not gotten to enjoy it myself because the separate volume knobs for left and right channels/ears bothers me so much, that I am left fiddling with it, and never get to listen to music, hence I am selling it. Some people are fine with the knobs, and don't find them very annoying, some people even find them handy in balancing the soundstage, such as Skylab, and it drives some people crazy, like me, because I can hear such subtle differences in volume. I can literally touch one knob with my finger and I hear the difference. I don't know if you would like it, but if you don't, resell it or talk to me and maybe we could arrange something.
Classified Listing: http://www.head-fi.org/t/565701/darkvoice-337-with-many-extra-tubes-575-obo#post_7684414
Make sure to look at the link I put in there for a better description, pics, list of tubes included, etc.
I would guess that this would drive the HD800s fine. You will want a fairly powerful amp for HD800s though since they are a flagship headphone. I wouldn't recommend a budget little <$300 amp (price does not always equal power or quality though, I am saying just generally). I had the E7 and it sounded like crap with my 650s. No imaging, everything sounded it was directly on my right or left side, and far away at that. Not very powerful. It also broke, and it took me a few weeks to get an email response from FiiO, and it was in frickin' Engrish!
Edited by jtaylor991 - 8/22/11 at 6:41pm