I forgot about Svetlana, yeah they have a good reputation too. It's too bad tubes cost so much more than a set of strings, so it's not economically advisable to just try different ones out...but you're looking at around $60 for 4 tubes for a 100 watt amp, or more, while I get guitar strings for around $5 a set. Quite a difference...
I'll be replacing preamp tubes first, all my power tubes are fairly new, so I'll be replacing a half dozen 12AX7's but then I have to get at least 4 spare power tubes, preferably 6...2 in the Super, 4 in the MX, 2 in the Classic, but it needs repair so it's not an immediate issue, but if I ever happen to have to replace everything at once, I'm looking at a big bill...
I think one of my preamp tube replacements was a Svetlana, can't remember though, i've slept since then...
Might have been Sovtek...