I respectfully submit that if someone changed 12ax7 tubes and the listener was BLIND (= listening only with ears) and didn't know what tube he was listening to, it'd all be a sonic blur with differences no more significant than bugs on the proverbial windshield. I have a box full of recent & vintage 12ax7, 12at7, 12au7 tubes and have messed with 'em a lot, in numerous heads/preamps, and have heard only minimal, subtle (even equivocal) differences between them. I realize there is an entire industry around/cultish belief in the different sounds of different tubes, but IMHO (again, respectfully), to the truly BLIND listener, the differences are marginal at best. If someone replaced one of the tubes in one of my Sushi pedals with another of the same type, there's absolutely no way I'd power it up afterward ........... and gasp in disbelief because my tone had changed. I'm not saying there are no differences, just that those differences are too subtle to distinguish with only one's ears.
It reminds me of vodka. I told a couple of friends who are very particular about their vodka brands that there was effectively no taste difference between double distilled vodkas, and that by definition (because there is an accepted standard), vodka is distilled to the point where the impurities (which are what actually impart flavour) have been removed, effectively leaving nothing but ethyl alcohol. They swore up and down that their Grey Goose/Chopin/potato vodka, etc. was far more delicious than the local bargain brand.........I offered to wager that in a blind test they could not reliably distinguish between their 2 favourite boutique vodkas and 2 cheapo (but double distilled) local brands.....they declined.....so I then suggested we do the test with no money on the line.......they had no rational reason not to try that, and unsurprisingly, none of us could distinguish between any of the 4 vodkas when blinded. Psychoacoustics are a thing, and tone chasing is the most fertile ground for 'em