I would flip the switch as the M80 are rated at 4ohms. this is likely a limiter of some sort so that you don't over drive the power supply by pulling more current that it can handle. ie it might limit the volume that you can get so you don't melt the receiver.

Anthem: AVM60, Fosi DAC-Q5
Axiom: ADA1500, LFR1100 Actiive, QS8, EP500, M3, M3comp, M5