I heard this amp a few weeks back. It is truly special, a real smooth sound with awesome bass and that tube mid range I love so much. It ain't cheap, granted, and it is a kit (though complete kit). I takes about 8 hours to build.
The parts inside this baby and the sound on offer, I would rate it as one of the best tube amps for this kind of headphone. The output transformers play a big part in the wide and dynamic sound of course. I think it is fairly unique as far as the output transformers are concerned?
Anyway, I DO recommend looking at this amp, it is truly special if you are after smooth fatigue free listening, top end extension with no edge and great bass. It is compatible with a whole raft of other headphone types (not electrostatic), so is very versatile.
Ping me if you want to ask anything about the sound. The company that make these amps (audionotekits) is an off-shoot of Audio Note UK, and is quite famous for pure tube based amps.
I have a Stax SR-007 set up which I love, but the HE-6 and this amp was very close as regards outright SQ, but in a different kind of presentation. I kinda prefer the HE-6 and this amp for electronic and rock as it gets the blood pumping, and the Stax for orchestral and vocals. It would be real nice to won both, I wonder if I can find an excuse to sneak it in?
Here are some pics of their website: