I’ve bought a few sets of power tubes from Viva Tubes, and they’ve all been good. They’ve been low on Russian power tube stocks for months, but still have some of those Sovteks, plus EH and Mullard KT88. You can also use KT88 or KT90 in the Ref 6 btw. I’ve heard EH KT90 is the best sounding choice (I’ve used Gold Lion KT88 in Ref 6, but not an EH KT90 myself) but you’ll have to run with acrylic top off due to height.
I’ve also used Brent Jesse GE 6550 in the Ref 6 and those are excellent, and reasonably priced. The Sovtek 6550 is OK but I’ve preferred the other tubes I’ve tried.
I bought some EH Gold 6SN7 from Tube Store a couple years ago and they almost immediately sputtered in my amps. That’s soured me on them as a source.
Upscale Audio tubes have always been 100% reliable and I’ve bought a TON of them over the years. If you’ve been a regular customer of theirs in the past, give ’em a call and they’ll work with you.