There are a lot of ways to work around things. For example, I read enough about rectifier tube sagging, and all those. All I have to do is to put a series resistor on the +B line, so when the amp draws more current at high signal level, more voltage drops across the resistor and sag!!!! There's no difference from sag by the tube, by the transformer or a resistor!!!! They sag!!!
From experience of designing a few guitar amps, you change the value of one resistor in the tone circuit, or one cap, you change the characteristics of the amp. There are so many critical components that change the sound, the transformer is not one of them from my experience.
This is a typical tone control used in Fender, Marshall and many other amps. People are just copying from each other. BUT by changing R1 from say 100K to 50K, you make the amp graw, changing C3 change the sound again. I put the value of Fender, I believe Marshall use 0.022 for C2 and C3. Fender has a more refined sound, more bass because C2 is 0.1. Marshall has a more brawy sound with less bass because C3 is 0.022. I have a switch in my amp to parallel another resistor to R1 to lower the value so it's better for rock music, or leave the 100K to have a more refine sound for other music.
If you use 50K for VR3, it will make the amp very mid heavy.
These are where one work on the sound. Use the resistor to create sag. I use power scaling to turn the power down so I can get to the sweet spot of the amp and any power output level.
The made of particular component is the last thing I worry about. I buy the cheapest of the cheap Shinghan tubes on ebay, I use all ceramic caps even in the tone circuit. I use all metal film resistors. I intentionally went against all the conventional things people said. I try to make a statement about this. I hope you can listen to my amps.
I tried changing the OPT from classic tone Bandmaster to Weber Plexi 50W, no difference I can aware of.
Go experiment the tone circuit yourself.
I have one amp using 6V6GT, another one use EL34......I think!!!! There's no difference. I use two different type of speakers, that change the sound of the amp completely. I like the Warehouse speakers.