I care mostly about noise and microphonics. Channel variations are what a balance control is for. Phono stages and preamps in particular deserve the quietest tubes you can find. I would use only the tube type specified by the manufacturer. Euphonic distortion is just that, distortion.
Do small preamp tubes 12ax7 need to be matched?
I had a Telefunken 12ax7 tube go out on me in the V4 (R) position of my Aesthetix Rhea, so I replaced the pair with stock Sovtek 12ax7WBs. Do I need to replace the Teles with a matched pair, or can I just pick up a single and throw it in. Obviously, I'll stick with smooth plate, as that is what was in there.