I think NOS tubes are an area where who you're dealing with matters more than anything.
My experience has been that tubes matter a lot more in a preamp than an amp. I've had tubes fail in my amps, but I've never experienced a sound quality issue related to a tube in any of my amps.
On the other hand, my Modwright LS100 which also uses 6SN7 tubes will not only reveal the characteristics of the sound quality of any given tube, but also noise and/or microphonics. I would say that probably 30% of the tubes I've purchased, including some highly touted new ones, have been microphonic or became microphonic after a short period of time or had unacceptable noise issues.
Which goes back to my initial point. Brent Jessee has been my go-to guy. For a few bucks extra he will guarantee his tubes (for 2 years I think?). Whenever I've experienced an issue, he's been great about getting me a replacement, or if he doesn't have one, recommending something different. I've also had good experiences with the ebay sellers I've bought from, but I make sure that they have 100% feedback and a lot of it.
I had some issues with a pair of tubes recently and the seller tried a few in his preamp and sent me four to try and told me to keep the two that worked best. I've had other good experiences with ebay sellers helping me make sure I got a good tube(s) that would work well in my preamp.
As far as buying a pair of NOS tubes that don't exactly match as far as their markings, that is not unusual, at least in my experience, nor does it concern me. The same tube may have been sold under different brand names. As long as the tubes themselves are the same and tested and matched, it makes no difference to me.
