I'm never good at diagnosing these.

When I have the amp on but the receiver off, only the M22 center is making the hum. The M80s are just putting out a faint hiss. When I turn the receiver on, they all make the hum. When it's just the amp on and not the receiver, the hum does follow the M22 if I switch what channel of the amp it's on. What could this mean?