I have experienced hum from dimmers even when speaker cables were disconnected at the amp and the conductors twisted together to simulate the output impedance of the amp, close to 0 ohms. Fixed it by installing twisted pairs for the cables similar to professional speaker cables from Belden, Mohawk and others which reject common mode noise.
Speaker hum
Would going from a 90db sensitivity to a 98db sensitivity speaker uncover a hum? I've been doing all the plug / unplug / cheater plug moves and haven't found it yet. It started when I swapped in new speakers and it's making me crazy. I have a 2-20 amp duplex receptacles fed directly from the breaker panel, same buss bar and grounds tied together in the 4 x 4 box at receptacles. I have an Odyssey Stratos Plus stereo amp and a NAD - C 658 BluOS Streaming DAC/Preamplifier.