With a properly serviced amp; upgrading tubes, speakers and other components can improve tone quality. There are also preferred circuits that can be referenced to "back date" your amplifier and popular non-factory circuit changes to customize the function and tone of your amplifier. Previous changes can be re-done to return your amp to stock.
Marshall -Most tube amps, current and vintage
Vintage Fender
Fender Hot Rod Series, Blues series and re-issues
Mesa bias control