Good question. I have been investigating power tubes for awhile, and am considering:
Tung-Sol 6550 Black Plate (duh, the "real deal" bend-me-over) NOS
GE 6550A if I can find them NOS
TS6550 Russian Tung-Sol "reissue"
Svetlana 6550C or whatever they're called now, which I have not heard yet in my amp (new prod.)
For my preamp, I think I'll probably buy a smattering of tubes and see what I like. 12AX7's, I need 3 of each variety. I haven't really started checking into these yet. Right now I run 12AX7EH which have warm and punchy bass, an excellent midrange, and extended if maybe a touch of sizzle in the highs especially on some digital material. I would like a more balanced approach, with better highs, maybe even at the loss of a little extension. I have been intrigued by the 5751's and will most definitely buy a set of these; probably some Sovtek 5751's to see what I think of this lower-gain tube.
I'm thinking the best bang for the buck would be some Svet or TS6550's in the power amp with some NOS in the pre. Toss in a smattering of new-production 12AX7's, also, for fun (and since they're relatively cheap). Any suggestions?
Also--I noticed you use a MF X-CanV2. I am considering the V3 version... How do you like yours? I just bought some Grado SR80's; I don't use headphones much anymore, but plan on using them when my wife watches dumb TV shows and the hifi is tied up