Thirty years ago you could find me installing lots of popular guitar amp mods -  master volumes, channel switching, effects loops, gain and tone mods, etc.  Now these features are often standard, and classic amps are too old or valuable for risky tinkering. 

Certain non-disruptive modifications to a guitar amp can make sense but mostly I focus on keeping gear running as original.  If your amp's sound or features don't fit the bill you should consider getting something different.  Over the last 20 years, I've done more de-mods than mods.

Don't overlook swapping basic elements like guitar pickups, power tubes, or speakers.  These can alter the sound of your amp a lot more than most circuit tweaks.