Well it sounds like you can help cure my nervosa.
More like pouring on gasoline!
I have a Belles 22A preamp that uses 2 12AU7s. I believe they currently have vintage RCA 5963 tubes installed.
3 questions
Are the RCAs good tubes?
What other tubes would you recommend for rolling?
What's the lifespan on these tubes? I leave the preamp on a lot and I'm concerned about wearing out the tubes.
Genelex Gold Lion 12AU7 would be my first choice for new production tubes.
Belles 21 is an all-tube preamp that came with lousy "Dayton" output caps. The 22 is solid state output stage, but you should check if there are any lousy coupling caps in the circuit. Upgrading those would yield bigger improvement than rolling tubes.
The tube life depends on how hard it's used. Belles is not hard on tubes. You should expect 5000+ hours with good quality new tubes with typical B+ and bias current. 2 decades of light use, or a couple years of always on. You'll hear when they get old, it sounds wimpy, syrupy, soft, or harsh and lots harmonic distortion like ringing.
RCA made excellent quality tubes. They tend toward slightly warm sounding, very musical, classic vintage tone. Not as syrupy as Amperex or lesser extent Mullard, but usually not as detailed and clean sounding as most current production tubes or vintage Telefunken. But there are some RCA variants like "cleartop" that are more detailed, at expense of less bass. I have a pair of these (Black Sable cryo from tube depot) that I'll probably never use, if you want to try them. With your solid state output stage to support the bass they might be good, idk. Shuguang 12au7 is very inexpensive and very clear, clean, good extension, full tone, but probably not as refined as the Genelex.