Modding an amp could be as simple as upgrading the fuse in the amp to a larger size, to cutting out the protection circuitry, to upgrading the major internal components of the amplifier.
IMHO, modding an amp for more power in a daily driver vehicle isn't worth it. It would be much better, and safer, to just buy a larger amplifier with the power you want/need. Majority of the time the "modding" is removal of the protection circuit.....not something you don't want to have in a daily driver amplifier where about anything can go wrong at any given time and possibly damage other equipment in the vehicle.
Only time I'd mod an amplifier is for 1) competition, or 2) if it's a factory authorized modification such as Linear Power and Tru Technology do. Other than that, it's basically a waste and dangerous. In my opinion atleast.