Change Can Be Good
Often the best guitar amp modifications are the simplest ones. Things like trying a different brand of tube or using a higher quality speaker can make a significant improvement in tone and not be very costly.
In vintage amps, certain types and brands of coupling capacitors can enhance or alter tone quality to suit your style. Feedback can be altered or made adjustable to provide a more dimensional and responsive sound. Some later model Fender amps can be “blackface modified” for better dynamics and tone. Marshalls made for the US market with 6550 tubes can be converted to EL34 tubes for an original British flavor.
I will not do any mods that adversely affect the value or reliability of an amp, or that cannot be easily reversed.