I hope the originator doesn’t mind taking this thread on a detour.
Gerry is the only guy i know who has had all of the top end tubes and he has a nice audio set up.
Sound quality, build quality, warranty and ultimately cost performance is how I buy fancy tubes now. But when a pair of tubes costs as much as a new amp, I find a compromise that my wallet and listening enjoyment can both be happy with.
300Bs, even the affordable Russian made EH last a long time when used with a conservative operating point. My EML’s are eight years old and test nearly new. Hell, they may outlast me.