20 amp cables for PS Audio P20?

Sir, you’re fine with the power cable that you have.

If the PS20 has a 20 amp IEC than use a 20 amp cord, if not use the 15 amp cord.  Don’t make it harder than it needs to be. I would use at minimum a 10 gauge cord. 

The P20 has the ability to be fed either by a 15A cord or a 20A cord, properly terminated. There are two AC inputs on the back. Only one can be used at a time. When used with a 15A cord the maximum power the P20 is able to manage is limited by the 15A circuit feeding it.