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.