I see! OK looking at that unit, it doesn’t appear to have a separate ground. Although it does seem to be a low build quality which could be causing your issue. If you wanted you could try running it on DC power since that would be filtered, it could fix your issue.
Are the in wall speakers on a different amplifier?
sem@piefed.blahaj.zone 2 days ago
Yes the house speakers (almost imperceptible hum, with the wires in the wall) are amplified by an Onkyo Tx-8011, which goes through a Niles audio HDS-6 high definition speaker selection system, with the protection toggle always engaged.
For protection, it says:
Disengaged:
1-2 8 ohm
1 4 ohm
Engaged:
3-6 8 ohm
2-6 4 ohm
But honestly I don’t have the users manual and I don’t know what it means.
Back to the test setup (with the louder hum; speaker wire NOT in the walls, just draped over the dining room table), the speakers are AR18s, U028144. The 2 pronged plug is NOT polarized. I’ll look for a DC power supply and see how that goes.