I had considered the WA6SE, as it was one of the few tube amps that is supposedly a decent match with the AH-D7000 (TH900's predecessor). However, I'm considering purchasing this new amp.
http://www.musicalsurroundings.com/Fosgate/fozgateheadphamp.html
While the output impedance of WA6SE is measured 30 ohms in low gain output (which is pretty high, considering TH900's impedance is only 25-30 ohms), the Fosgate Signature Headphone Amplifier's output impedance is only 4.7 ohms. That should provide a better damping factor than the WA6SE. Plus, it has a pair of RCA outputs as well as inputs, which means I can use my Emotiva airmotiv 4 desktop speakers with it too. Lastly, its bass boost and surround sound feature seems quite interesting.
It costs $1,500, which isn't really cheap but moderately priced compared to most high-end tube amps out there. I'll try this FosFos (Fosgate/Fostex) combo and share my impressions.
For the record, as you may have guessed, this amp is made by none other than Jim Fosgate, founder of Rockford Fosgate and creator/inventor of Dolby Pro Logic II surround sound technology, the latter of which won him a special Emmy and is incorporated into the surround sound circuitry of this very amp.