Modifications to production amplifiers are quite common. Modifications can be done to change the tonal characteristic, gain, or fundamental functions of an amplifier such as adjustable bias.

Not all mods are created equally even, and there can be a trade off with a simple mod against a  loss. 

A classic example is that a tone boost can cause a tone cut elsewhere. 

A Master Volume modification can change the tone of the sound from quiet to loud. 

If you are going to research about Mods on forums it is very important to understand not only the good, but also any bad also associated. The good elements often outweigh the bad elements however are not often discussed. 

If a Marshall was made to sound more like a Fender on clean, and more distorted like a Mesa, Marshall would have made it like that.  There is a certain level of respect needed to be given in why amps have their function such as they do, the designers spend much time trying different drives, distortions, EQ to get it right for their signature sound.