Why would you want to change the power transformer? All it does is take the AC wall voltage and steps it up for the high voltage supply that feeds the tube plates and steps it down for the tube heaters. There's only a few things that can go wrong: mechanical vibration, electromagnetic interference/coupling with adjacent wiring/coils, and not having the proper current/voltage rating for the amp. I have a crack, two Stereomours, a BeePre and a Kaiju -- all with stock PTs. Never had an issue with vibration or EMF intereference/coupling with any of them -- and the stock PT is obviously suited to the voltage/current requirements of the amp.
cheers, Derek