I've had a sextet of Svetlana 6550s in my SVT-II for about four years now. They haven't seen real heavy use--regular rehearsal use for probably 2-3 of those years, and maybe about 15-20 gigs total--but I'm getting pretty weak volume now. I have to turn it up past halfway to get it even kind of loud (through an SVT410HLF cab), and they are breaking up about halfway up the dial (that part could be normal I guess). Seems like they should have lasted longer, but I get the impression these are budget 6550s. Does anybody have any recommendations for power tubes for these amps? Current production, please, unless you can find a NOS tube that sells for ~$200 a sextet.