I just got some Meridian active loudspeakers (analogue): M60 mains, M60C centre and M2500 subwoofer. Hauled out the old Yamaha RX-V1400RDS receiver and connected them up. The sub is behaving, but the two main speakers emit a constant-level hum as soon as they are connected to the Yamaha's line outputs. When disconnected, the speakers don't produce hum. When I switch off the Yamaha the hum becomes loud. Any ideas what could be causing this? I have not connected the centre up yet.