I've got the HP-A8C, it pairs with the th900's very well - albeit I think it's a tad short on power (approx 700mW @32 ohms). I tried out the Burson Conductor with ESS Sabre and found it's amp section to be fantastic! The HA-1 uses the same DAC, and has almost the same power - but with lower impedance.
I'm in the same boat. I just re-purchased the th900's for a bed-side rig, and am working through amp options.
HP-A8 - pros: great DAC, already have it. Made for th900s. cons: meh amp power, would have to rob my office setup.
HA-1 - pros: powerful & clean. Looks great, low impedence. cons: sabre DAC is detailed, but very neutral. Would need to re-terminate balanced to get full power.
Burson Conductor with Burr Brown DAC - pros: great amp section, burr brown dac is warm & musical. cons: dac isn't as detailed as sabre. 25-step attenuator isn't great.
Burson Conduction Virtuoso w/ BB DAC - pros: great amp, warm dac, 100-step attenuator, remote. cons: same as above, except for upgraded attenuator.
*note: Burson offers swap & play DAC boards. $350 for the Sabre, $250 for the Burr Brown. It's essentially cheaper to buy the BB version and get the Sabre later.
Off the list:
Peachtree Decco65SE - pros: resolving Sabre DAC with tube in signal path (warmer sound), good clean power (1200mW @32ohms), drives speakers too. cons: headphone output impedance too high for th900s (it's 4.7 ohm, and the th900's need 1/8th of 25ohms, or less than 3.12ohm).
Woo Audio WA7 - pros: nice tube amp, small, handsome. cons: not the most resolving DAC, not a step-up in power (approx 750mW @35ohm), is better with more costly tube power supply.
Chord Hugo - pros: DAC gets lots of love, small, trans-portable, good battery life. Super expensive. Looks like a toy (IMHO). Many question it's DAC abilities & cost/value.
Might look at again:
CEntrance hifi m8. I actually have no idea how much power this puts out (not clear by website), nor what DAC chip it uses. Does full power come SE, Balanced, or both?