Do tubes matter beyond gain stage on preamp?

Not sure what you mean? All tubes matter.

Do power valves impact the sound? Very much. Low gain stage amps normally have a lower noise floor. That’s the reason why.

Macs, you better pay attention to the small valves. Every step up is a potential to amplify noise to the next stage from ALL those before.. The power valves take it one step beyond.

Power valves reveal every mistake or noise up to that point.. SO add a set of poorly matched or manufactured power valves, to an already noisy preamp section. It can get ugly!
I’ve purchased amps just for that reason and the fix was "matching valves" ONBOARD already.

Do just the opposite, paying attention to matching, balancing, and checking for noisy or vibration prone valves, you’ll appreciate a great valve amp vs hating a noisy pile of CRAP.

There are guys like ME buying perfectly good valve amps at 1/3 the cost.. I bought a MC240 for 1100.00 two years ago. I swapped some valves ONBOARD and replaced one 12BH7 (?) It was a weird one that usually don’t go bad. It sounded like it had a fresh rebuild.. I even offered to sell it back for the 100.00 I spent.. "A deal is a deal", OK I offered..

Chicken feeding time....

Regards

This may only have indirect relevance to your question,@emergingsoul, but I’ve been playing with different rectifiers for the power supply to my phono stage, an Allnic. Right now, I have a Mullard (really a Philips) metal base GZ 34 and a GEC u52 that I’ve been swapping in and out. Significant difference in sound between those two tubes. The metal base has incredible bass and dynamics but is slightly flat sounding in the high frequencies. The GEC (which I have used in the past) does not have the impact of the Mullard, but has far more finesse in the upper registers. I think the choice may come down to how the rest of the system is voiced. Given how my system is presently "tuned" the GEC is my preference.

I swapped out a neon voltage reference tube and it had a profound impact on the sound. So, yes, all tubes influence the sound.

Everything matters.