I admit to being a bit of a 12AX7 snob, and I also admit that this is a marginal, not essential, pursuit.
That being said, I put some old pre-war Yugo EI 12AX7s in the 3 slots in my Synergy 50 head (running TDLX & Bogner Ecstasy Modules) and the smoothness and lushness just reached another...