I've used quite a few. I like NOS telefunken 12ax7's, which are very smooth, but the newer Electro-Harmonix 12ax7's that were based on the telefunkens come quite close in terms of tone. In my triaxis, because it got thrown around alot, I used military-spec JAN Sylvanias for awhile, which decreased my gain a little but they never ever failed.
It's important to remember that Mesa voice their amps with the tubes that they have access to for production. So putting in vintage tubes isn't necessarily better, just different. Also, a lot of the mojo of boutique tubes really applies more to vintage or vintage-style amps, where the gain and breakup characteristics are much more power-stage dependent. Once you are using a cascading preamp style amp which produces most of the gain and distortion pre-power stage, it almost becomes silly to get anal about tubes. (But I did it anyway so who am I to judge).
In my experience, Chinese tubes tend to be bright and fizzy with more gain, russian/eastern european a little rounder. NOS military spec tubes (JAN Sylvanias for example) tend to be a little less gainy, round and fat, and quite indestructable. But hey, tubes are tubes and they all vary a little. And of course, always remember that most of your tone is in your hands anyway. I've seen Eddie Van Halen play thru the high-gain channel of a little Polytone jazz amp, and he totally sounded like Eddie.
