That is a question for the ages!!
I would recommend you do lots of research before considering having any modifications done to your amp. That being said, most gain mods will not sound better than using an overdrive in front of your amp. In fact, that's what most of them are - an additional gain stage added to the preamp stage of the head.
I have had one mod made to my JCM800, and that was to the power supply. I still use an overdrive in front of my amps (Fulltone Full-Drive 2) because I like the harmonic overtones it adds to the tone, as well as the general thickness it adds, as well.
Do I like or recommend the mod I had done? Being an audio geek, I really like it - the background is blacker, and the amp has a greater dynamic range. This mod was made by Harry Kolbe, who is no longer in the amp mod business, as he is now doing Atomic Amplifiers. Cosmetically, the front and back panels are preserved.