Here is a very good schematic showing a tipical power amplifier grounding system. I have tried it ina many of my projects and it works every time. No hum, no ground loops no noise. Dead silent. Of course a proper power supply routing on the main PCB is critical. If done properly some very very high performance could be achieved.
The schematic was proposed by D. Self in his book “Audio Power Amplifier Design”. For any further info, please check out his work.