I also like having less compression in rhythm sounds and more in lead sounds - that usually involves having fewer tubes working harder (filtered, of course, to reduce unnecessary fizz) in the rhythm section and more tubes getting driven less in the lead channel, but since I don't use tubes I've never had to worry about reworking preamp sections in those things.
Another big seller is transformer replacement - especially in cheaper tube amps. Again, not something I can say much about from my own experience, but I see sales tickets and that seems to be a popular one.
Jeff