After doing my first long session with the THX00 and ECP Torpedo III amp, I can confirm its a phenomenal match. Switch it to the low-Z setting and it really lets these babies shine. It really pushes these bio-dyna drivers to their max as far as bass response, dynamicism, extension on both ends and smoothness goes. It's the first tube amp ive really liked with any of the Fostex bio-dyna series. It doesn't change the sound signature very much, if at all, but it does remove almost all the high order harmonics and thus makes the treble much smoother. Bass is tighter than even with a zero impedance solid state amp, and hits a bit harder. Midrange is pure liquid bliss. Highs are very well extended but never fatiguing. If you find the highs in your THX00 fatiguing but you also don't want a dull roll-off either, this is your solution.
It's probably overkill for the THX00 at ~$1500 built to the specs I have mine at and shipped from Beezar, but if you're pairing the THX00 with an HD800, this could absolutely be your summit fi amp. Equally brilliant with both, which two weeks ago I thought finding a single amp that took the HD800 and THX00 to their max ability would be impossible. Doug is a freaking genius. Was talking to Purk about this yesterday, and we both think Doug is probably the best amp designer in the world.
For reference I have my TIII with: cinemags, Tube CCS, Output CCS and Mundorfs. Currently using the EH 12ay7s. But rolling in others too.