Is the sound coming from the speaker cones or from the transformers in the plate amp? If it's transformer hum it comes from DC on the AC power caused by load imbalance. Plasma TVs and light dimmers can cause this. If its not coming from the speaker cones, it's not a ground loop.
This product solved the same problem for me:
https://avahifi.com/products/humdinger-dc-line-blocker
the Emotive CMX-2 should also work.
This product solved the same problem for me:
https://avahifi.com/products/humdinger-dc-line-blocker
the Emotive CMX-2 should also work.