Shorting it as I described would be like your receiver putting out zero volts. No harm will be done.
Since the sub is humming without the cable connected, you may have airborne noise entering through the RCA jack. If you short the input and the hum disappears, that means you've shorted the airborne noise. If it still hums, it's likely a leaky power supply capacitor.
BTW, noise figures for audio equipment are always given with RCA jacks shorted as I described. The manufacturer wants to measure internally generated noise and not what might be generated by the ambient environment and radiated or conducted into the equipment by inputs and outputs.
Last edited by Mojo; 03/06/21 05:02 AM.
