The power transformer is wound for a 5U4GB rectifier which means it can handle the extra filament current of the 5U4GB.
The PT is wound to compensate for the additional voltage drop across the 5U4GB rectifier tube by having a higher HI-V AC potential from the start.
I don't know where these people are who write/say that they hate the 5U4GB and if there are any, they're stupid IMHO.
It is a very good rectifier tube and can handle twice the B+ current of any GZ34 and a higher AC voltage too.
I suspect that most amp "gurus" who might say they hate the 5U4GB don't or didn't know how to fix/mod their silverface Fender amp's bias circuit and then bias their amp's power tubes correctly.
Here's my take, if it what you think you've heard about the 5U4GB is remotely real,
...because many silver face amps are biased cold from the factory and most of them do not have a real adjustable bias supply, when using the GZ34 and not altering the bias supply in any way, the B+ rises to a higher level, taking idle current up with it, creating a fatter tubey sound.