Schematics are not easily available for your Parasound amps so I can’t confirm, but I wonder if the A23 is a fully balanced design and the 2125 v2 is not. This would explain the different results when connecting the amps the same way.
Try connecting the A23 as differential/fully balanced per the REL manual (page 15 in my Serie T/x Operating Instructions). That’s Yellow to the Left + terminal, Red to Right + terminal, and Black unconnected or connected to ground. This is how I connect my differential/fully balanced amps to my REL T7/x without any hum.