I enjoy servicing and rebuilding amps, my favorites are precircuit board Fenders.
To reliably use current production tubes, upgrades must be done in the power supply and output stage. Preamp sections automatically get new plate load resistors and cathode caps. At this point, the amp is consistent and reliable and can be tuned to the players taste.
As not all components are created equally, I find it is important to get a balanced mix of capacitors in an amp. Years of rebuilding amps have determined which components are reliable and sonically pleasing.
Transformers are another area that sonic gains can be made. Mercury Magnetics transformers are hand wound and sound superb. I use Mecury Magnetics transformers in all amps I build.
Many current production Fenders and Marshall’s will benefit by having a MM transformer installed. I also recommend that a qualified tech does the installation as there is a little more to this than meets the eye, plus lethal voltages are involved.