While setting up a multiple speaker test on the EA35 in-wall amplifier we unwittingly used a faulty mini jack lead which was open circuit (on all wires !!) the mains hum level that resulted when we turned the amplifier on was very loud (>110 dB) and louder than the test tone (the test area had lots of dangling mains cables for the test equipment and power supplies etc not unlike the mess found in some false ceilings that I've seen)

The hum was just from the pick-up of mains interference with no direct connection.

There was no detectable hum at all from the amplifier without a cable connected to the input nor with the correct cable connected whether or not it was plugged into the tone source

It just reminded of some of the "faulty amplifier" reports that we receive

So the message is always check the cables first ..... the cables are much more likely to be damaged when snagged during installation than the amplifiers.

Incidentally the tests proved that 2x 8 ohm or 4x 8 ohm speakers work really fine on this neat Class D digital amplifier with twice the "loudness" on the 4x 8 ohm (>3 dB)