My favorite for my HD800s is the Singlepower headphone amp with 6SN7 tubes installed.
A Singlepower would need to be purchsed used. (and a warning about the 'all over the place' quality of Singlepower products. Some were made in a questionable manner, with point to point wiring (no circuit boards), wrong caps, or inferior transformers.. etc. So IF you went for one, you need to see a photo of the interior of the EXACT one you are buying. If it is really nice and orderly, with beautiful wiring and a full circuit board.., great, if it is a rats nest, then pass it by.
The second choice i use is my phone output on the Bryston BP-26 preamp.
I also own a Rudistor RPX-33 mk II and do not use it with the HD800s. They both emphasize the upper octave a bit, and even though the sound combo is the clearest of any i have ever heard, and I can hear stuff on Cds I never knew was there.. it is not a great match up. (It may be a power problem, I have just not experimented enough with the Rudistor, i use several powerlne conditioners, and the Rudistor/HD800 combo may be best without one. never did bother to check it out.)
Then i own a Lil' Dot mkIII and it is a decent headphone amp. Ditto the TADAC i own.
I also own Beyerdynamic DH990, and Sony diamond diaphram earbuds, and a 25 year old pair of Stax electrostatic phones.
