Stock it comes with Chinese military spec 6C19 power tubes and Ediswan EF92 drivers. Not the most common tubes out there, but after reading a bit on head-fi about equivalent tube types, I did a quick google/ebay search and came up with some possibilities for both.
Also realized that you can remove two internal jumpers that supposedly allow the amp to drive lower impedance cans better. I dunno if I like the changes to the sonics, though. Bass actually tightens up well, but it loses something in the midrange.
What a weird world, this tube amp stuff.