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Currid-
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
Every setup is different, but I had hum with my powered sub 2.1 MiniDSP system. Solved it by grounding the sub end of the phono signal lead. Mains plug with a single wire going from earth pin to the phono cable shield. Seems sketchy, but I can't see how it would actually be dangerous.
Once I got a hypex amp with balanced inputs the hum disappeared anyway. I'll sell you a magik snakeoil coated plug with bit of bell wire sticking out for only $789.00 (m8s r8s)