Alternate Power Tubes for a Cary V12R

Howdy - I've got a Cary V12R that takes dozen EL34 tubes. The tubes are getting a little tired and I'm thinking about replacing them. I like the way the amp sounds, but I'm always looking for a little extra - a bit more extension in the treble and tighter bass. the Cary manual states that the amp can run on 6L6, 6V6, KT-66, KT-88, KT-90, KT-99, 6550C, and of course EL34. I'd like to retain some of the EL34 mids if possible. I'm trying to keep it to a dull roar, maybe $50-$60 per pair. Does anyone have any experience rolling tubes in this amp, what are you using, and what do you recommend? Thanks!