Well, turns out changing the 15A female connector on my power cord is even easier. No heat shrink, all screw crimped to 3 conductors, so I will be buying a 20A female connector and changing it myself.
Case closed
Fully resolved outcome!
This amplifier power dissipation is 500 watts maximum and 230 watts at idle. Only 4 KT 150 output tubes and 2 6H30 tubes. Power storage is 520 joules. Pure curiosity, why does it have a 20 amp cable requirement? The circuit would never come close to this amperage. I wonder what the ARC 20 amp implementation rationale is. Interesting.
Charles