One of the main source of innovation in guitar amps (aside from bells and whistles like alternate channels and effect loops) are typically the "tone stack", i.e

One of the main source of innovation in guitar amps (aside from bells and whistles like alternate channels and effect loops) are typically the "tone stack", i.e. the capacitance of the tone control caps and their position relative to the power and pre-amp tubes in the circuit. The amplifier circuits themselves are mostly taken from RCA radio amps dating to the 1930s and licensed by Fender in the 50s.