Originally Posted by cotdt
there's also the Pass Labs amps, which are more euphonic.
I don't believe that Pass Labs makes a dedicated headphone amp, though. A lot of people use the Aleph and the First Watt (by Pass, but a separate company) F1 and F2 to drive the AKG K-1000. Using them for any other headphone (e.g. K-701) would be overkill. And yes, Pass' amps sound terrific. I built his DIY Zen Amp and Preamp (for speakers) a little over a year ago, and they sound terrific.
As for solid state headphone amps, a lot of good ones have been mentioned so far. I think Gilmore's designs are exceptional, and you might also want to look at the Dynamid/Gilmore Reference amp. It's a bridged and balanced Dynalo/Gilmore Lite, and before the Dynamite came along, was considered to be the best solid state amp. I've got all the parts for one, and have a window of opportunity to build it in a little over a week. Can't wait!
I'm also looking forward to hearing the new Cavalli-Kan Kumisa III that I'm gathering parts for, and I'm very, very close to pulling the trigger on parts for a fully balanced Beta22. I haven't heard either, yet. However, the folks over at Headwize and AMB Labs don't get excited over nothing. I expect both to be excellent.