Ground loop hum

Just installed my Anthem MRX300 with PVA5 and have a hum . When I remove the Sunfire mark 2 the hum thru the speakers disappears. When I take the Sunfire to my friends Emotiva set up there is no hum thru his Martin loagans. When I replace the sunfire with an Energy there is not hum. Previously with the Marantz Sr19 there was no hum thru the speakers either. All else remains the same. Any guidance?
Brian