Both agony and ecstasy, as many of you are aware.
Vintage 6SN7 are quite often problematic in preamps (except as follower outputs), due to noise and microphony. 6SN7 are usually totally awesome in power amps (as phase splitter / V1 or driver tubes). The later GTA and GTB variants tend to be much quieter (OK for use in preamps), but have less sweetness to their sound - they do make great KT tube drivers in amps, though.
I've bought lots of tubes from Brent Jesse and Upscale over the years, both great 99% of the time. They're human, so mistakes do get made (I've had a mistake from each) but these cases are the outliers; both sellers are honest so they'll work to rectify any issue.
I've also got tubes from Andy / VTS, and they are as advertised; the problem is a lot of his stock is RCA and (in the end) those tubes have never worked out for me sonically. For the better part of a year I had some VTS low-noise selected RCA 12AX7 in my VAC Renaissance SE phono stage, never initially questioning them (dumb). They were indeed very low noise, matched and strong. But the sweetness and warmth of my VAC was all gone. I started to blame the VAC! When I finally got back to rolling in other 12AX7 makes, it was immediately apparent the RCAs were 100% at fault - bright and lean sound, ugh. Thinking back now, I just don't like RCA - sure there are exceptions, but even with those, there's always a different make I like better.
Ebay is a crap shoot, as expected. I've gotten good and bad tubes there, in equal measure.
The modern Russian tubes are fantastic; the sub-brands each have their own sonic flavor, and they're easy to find in low noise matched pairs. BUT they still don't have the 3D holographic image or ultimate articulation and refinement of the very best vintage tubes. Their warmth seems a bit more 2D and less nuanced, somehow.