I have a Univalve (single-ended Class A tube amp) and played with lots of 12AX7s (ECC83s). My cousin rebuilds old vacuum tube stuff and even makes low-wattage tube amps (he gave me a whole bunch of vintage 12AX7s to play with). To be brutally honest, I didn't see a huge difference between brand new Chinese 12AX7s and vintage Mullard ECC83s.
I've bought some stuff from these guys:
https://www.tubedepot.com/t/tubes/preamp-tubes/12ax7-ecc83-7025-cv4004
Fun to look around...
Power tubes are a different story. Lot's a good power tubes that make a huge sonic difference...