A common way of wiring the RCA jack on amplifiers is to connect the RCA ground to both the signal ground and the chassis. The chassis ground connection is isolated through a filter with low value cap. The problem then could be the RCA jack is touching the chassis (a worn insulator) or the coupling cap from RCA ground to chassis has shorted. Try gently moving the RCA cable at the amp input and see if the hum changes or goes away.
Krell KSA 200s ground hum
I picked up this amp a week ago and I REALLY like it. When I first plugged it in with nothing attached except speakers it was silent. Like small hiss from an inch away but one of the more silent pieces I have had. As soon as I hooked up an rca from my preamp there it was. Loud ground hum. So right now I have floated the ground with cheater plugs on everything in the chain. Silence all is good with the cheater plugs and the system sounds great. If I take the cheater plug off of any of the 5 pieces in the chain it’s back! All same outlet and dedicated 10 gage line. I mean I’m “ok” with the cheater plugs if need be but…….. any ideas?