I'd look at the good solid state amps in your budget. The CK2III, M^3, Gilmore Lite, Dynalo (I think it's in production), and EC/SS all fit your budget with some left over. Those are all circuits from well-regarded designers around here (see the DIY Forum) and are very good amps. Other amps might be good, but look at the level of respect designers like Kevin Gilmore, Craig Uthus, AMB, Cavalli, Millett, and some I'm probably forgetting, get around here. I've tended to buy or build their amps because they're quite good and have been picked through by the people who really know electronics.
That isn't to say that other amps are bad (and some are probably excellent), but it's more of a "sure thing" to go with one of these well-known designs. Don't worry about them being out of date or not cutting edge. You don't want that, anyway. When you've got a design like a M^3 that's been built by hundreds of people over a few years, you know it's a good amp because any and all problems have been spotted and stamped out. If serious trouble or a performance issue comes up, someone will suggest a solution and it'll end up in the design. Also, amp technology doesn't change very fast. What was a good design 10, 20 or 50 years ago can still sound great today. This is why I take a long-term view of design, reputation and build quality.
Instead of spending $800 on an amp, I'd get one of these and look into spending the rest on a different pair of headphones. Or if you end up with $400-$500 left over, go to Audiogon and see if you can find a used Rega Planar 3/P3 in that price range. Instead of spending an extra $400-$500 on an amp just because you can, you'll get a lot more enjoyment out of a turntable or a different pair of headphones.
