My friend bought a tube preamp and tube phono stage, brought them here for a listen. I was quite surprised how much the sound changed when he tried the alternate rectifier tube the maker had included. Quite obvious difference, not subtle.
I bought a pair of 6sl7s, hated them, returned, tried different ones, love them. Again, obvious, not subtle.
Seemed like a bigger difference than I have experienced when changing 12ax/12au's