20amp v 15amp

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.

a good read: 15 AMP VS 20 AMP OUTLET

I think you should also consider the max wattage draw of your audio system.

up to 1800 watts for a 15 A

and 

up to 2400 watts for a 20 A 

investing in a good power conditioner can help with overall system performance. I've explained it quite a few times already.

Not really, no. 

For many manufacturers the 15A and 20A outlets are identical except for the plastic face plate. They literally use the same brass behind the plastic, down to the T shaped connectors.

Fact is 15A outlets have to live with the possibility of being on a 20A circuit, so they are built with that in mind.

It's a weirdness of the NEC that they expect you to run 20A circuits to multiple 15A outlets.

Get a good 12 Gauge cable, no matter the connector on the end.

If  the wall outlet is 15-amp 125V then you'll need either a 20 amp to 15 amp plug adapter or a one foot 20/15 power cord. Each are less than 10 dollars. But if you know how to replace a receptacle, then install a combo 15/20 amp 250V receptacle. Fits perfectly in the same box.

At minimum you need a power cord with a 20-amp IEC-C19 plug end and the other end with a standard 15amp IEC male plug. See AudioQuest Monsoon Power Cable (2 meters/6.5 feet) Power cable with 20-amp IEC-C19 plug at Crutchfield As a preferred upgrade, you should have a new 20 amp dedicated circuit installed (new 20 amp breaker, 12 AWG cabling and a 20 amp outlet. This would realize the full potential of the Audioquest Niagara 5000. The new power cable would continue to be compatible with the 20 amp upgraded circuit.

You most likely need a 20 amp breaker along with the proper gauge wire run to a 20 amp outlet.

Probly.