I have had this exact same issue and thoroughly documented my experience in this thread:
I have come to the conclusion that the P20 generates DC as part of its regeneration. The only way I could eliminate the hum in the large transformers of my amplifiers was to add a DC blocker after the regenerator (https://emotiva.com/products/cmx-2).
I loved the sound coming out of my speakers with the P20 connected. I preferred the sound of my amplifiers transformer when directly connected to the wall.