So, the "remote" where the volume control lives is acting as the pre-amplifier (kinda, it's more that it's restricting the "volume" of the input source). Signals come from the PC here and a potentiometer (the volume control wheel) determines how much this signal is restricted - full volume I am assuming is "no reduction in input power" . It's also where the majority of the brains are located, including some additional circuitry which allows for the speakers to be shut down when headphones are connected. And most importantly, when this happens the hum goes away.
This "speakers turn off when headphones are connected" function interests me. It's not like power is being cut to the speakers, just the input source. If you put the speakers close to your ears you can hear a faint hiss, which shows that the power amp is still powering the speakers but is not amplifying any signal.
Further testing also showed that the headphone socket will work even when the speaker system has no power to it at all (even at the mains wall socket). This shows that the headphone connector is mechanically switching the audio input source from the PC sound card to the headphones. Only when the headphones are removed from this socket does the mechanical switch go back to passing the audio input source to the X-240 power amp, via the volume control potentiometer.
Therefore, my deduction is that the noise is coming from the circuitry in the remote. The power amp in the subwoofer box has filtering capacitors on it and shows - by itself - no external noise source. At the moment, I believe that the circuitry in the remote is the source of the noise, or there is noise coming up the power cable from the woofer box.
There are a number of ways I can try to test this, and my ideas at the moment are:
- Solder in my own separate power lines between the subwoofer box and the remote
- Hard-wire the power circuit and use the switch on the remote just for the audio connection
- Hard-wire the audio connection between the PC, the potentiometer, and the output back to the power amp (bypassing the power switch)
- Hard-wire the power, and also use my own potentiometer for volume control rather than the surface-mounted one in the remote.