The bias mod and the choke are the only mods I think are necessary on the 5150 circuit. I feel like that is how the circuit should have came from the factory if Peavey wasn't cheap. Biasing tubes is important to getting them to work properly, and without it, the amp is sterile to err on the side of caution. The choke is just cutting $15-$30 in iron from the build. The choke really tightens up the amp. It is so much more metal after that. The only other common mod on those is to clean up the green channel. What the hell do you need a clean channel for?!