I pulled out an old receipt last night and realized I've been playing the same set of Visseaux 6V6's in my SFDR since 1990. I've played about 30 4+ hour gigs a year plus weekly / bi-weekly rehearsals that whole time. I'm thinking it might be time for a re-tube, however the amp still sounds strong to me, and I don't want to blow the money if I'm not going to get an improved tone for it. Any thoughts on how to check this out or what the "standard" time is for replacements? Thanks!