Here are some basics... I’d remove the RCA’s, listen to the amps with no input, fiddle with the gain, no buzz, at least you know it isn’t an amp ground loop problem. Try changing the source, any other source directly to the amps (making sure the output voltage is within range) using the existing RCA’s, no buzz, not the RCA’s. Make sure you are using twisted pair RCA’s and make sure they do not run parallel or close to your power/ground wires, cross paths only at right angles. If you have multiple channels and/or separate amps, pull all the RCA's out and try each one on each input for each amplifier, each one at a time on to see if one is the source. Not sure what you’ve tried so hope that it helps. I’ve never had very good luck with ground lop isolators, personally. I keep a cheap HU with 4V preamp outputs on hand to test stuff like this, worth the $50 to keep it on hand even if it only gets used for this purpose.