Hi - I know this is an old post, but I have a pair of these mated to a Mercury 3 pre. GREAT amps, the quietest tube amps I own(and I own a few, mostly for sale now....) They can run a LOT of different power tubes, but my favorite so far is the Tung-Sol 7581A, which is kinda the sweet spot IMO. I asked Kara about this tube, she didn't know it existed, but she confirmed later that it was a good choice. tHowever, and I feel like a dolt for saying this after having these as long as I have, but it turns out I've been under-biasing this whole time, so will have to rebias(incredibly easy, by the way) all the different options and see how I feel after all that. The other stupid thing I have to find out is whether or not they invert phase. I was looking into some of her other amps and it looks like all of them do. Doh! Let me know if you have any questions.