The main issue is safety and not overheating the power wires because the powered devices are drawing too many amps, which is a definite fire hazard. I doubt many amplifiers draw 20A all by themselves (i.e., I can't think of any with a 20A fuse). However, I suspect some amplifiers use a 20A cord because the manufacturer has considered that some users may connect the amplifier to a circuit with other stuff on it - IOW, because of the high'ish current draw of the amplifier, there is a greater chance that the total amperage of devices on the circuit may exceed 15A. Side benefits are that the amplifier's power cord will have a better connector and heavier wire.
Your easiest solution is to make sure the amplifier is plugged into a 20A outlet powered by a 20A circuit using at least 12awg wire and a 20A breaker. Do not replace the 15A outlet with a 20A outlet unless you confirm the entire circuit is rated for 20A. The reason for this is to prevent someone from plugging a device that draws 20A into a 15A circuit and overheating the wire in the circuit.