Loud Electrical Hum from Sub Preout when units are in Standby!

[ogs]

If this is 'normal' hum, then it is because the cable (and hence the subwoofer input) is not grounded when the Devialet amp is off or in standby, i.e. the Devialet disconnects ground of the RCA rather than creating a short between signal and ground. If you disconnect the cable(s) and touch the signal pin without touching the sleeve of the plug you'll probably get a similar (not identical) hum in the subwoofer. As soon as you touch the metal on the sleeve hum will be much weaker or go away completely.
The other type of hum can , as zdenes says, be created by the cable itself. The input circuit of the subwoofer is then triggered into an unstable mode. This is actually not hum, but a sort of oscillation in the input circuitry where the cable acts as a kind of antenna - when it is unterminated at the amplifier end. A different cable can help here.

EDIT. keeping the ground connected and disconnect the signal pin in the amplifier could also work. Either way it is something Devialet needs to fix