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your USB port on your laptop won't be connected to earth which means when you power it via USB you're effectively lifting the ground. This means that your connection to earth will be via your powered sub (I'm assuming that's grounded).
It could be that your sockets that you're using are earthed to different points. Try plugging everything into one extension lead which is plugged into one socket. If that gets rid of your ground hum then it's likely that the issue is with your sockets.
gorgonzolach
Currid
Ha, bargain.
What I don't get is why there is no hum when it's powered via my laptop?!