80amps x 12 volts = 960 watts... the 80 amp fuse was already more than enough, by putting in the 100amp fuse you just changed it to 100amp x 12 volts = 1200 watts, and thats assuming your voltages were at 12 volts... they were probably a little higher than 12 anyways. Anyone feel free to correct me if im wrong...
As for the gain, should never be anywhere over half, especially if you're not setting it with a DMM.