Just when I thought I was done unscrewing the box and disconnecting things.Do the amplifiers produce hum when the input cables are disconnected from them?
Yes, the amplifiers produce hum when the input cables are disconnected from them, but at a noticeable lower volume. With Bluetooth powered off and one set of input cables disconnected and the volume at 100%, 15 seconds of my iPhone's decibel meter reading with the phone flush against the speaker = AVG 54.5 decibels. Same test with the wires connected = AVG 69.5 decibels.
Edit - the room isn't a controlled environment, so I'm not sure how accurate or relevant the decibel readings are, but it is 100% clear that the buzz remains with the cables disconnected, and it is also 100% clear that the volume of the buzz is most-likely at least twice as loud with the cables connected, then with the cables disconnected.
Second Edit - I'm guessing this means that even though the rest of the wires are shielded, the shielding isn't powerful enough to block all the power adapter leakage. Last night, when I replaced the 22 AWG insulated amplifier input wires with shielded audio cable, the buzz persisted. Too much of a pain to put those thicker wires back in for another volume test, but if those cables reduced the volume of the hum, it wasn't apparent, so if it did, it wasn't by a lot.