Help with ground loop

Need this forums help with a ground loop. System is oppo 205 to McIntosh c52 to McIntosh mc275-6 for two channel. Also run oppo 205 to marantx av7702 to McIntosh mc452 for 5.2 setup.  Never had issue until recent purchase of c52. The oppo connected to the c52 via balanced cables to the mc275 with balanced cables creates a hum. Connected c52 to mc452 with same interconnects, dead quiet. Ran oppo 205 direct to mc275 with same interconnects, dead quiet, so I know not bad tubes. All powered by a dedicated circuit with ground direct to panel terminal block. All sources connected to a zero surge power strip. Is it possible the C52 and the MC275 have different ground potential? Pulling my hair., please help.