Congrats, you found the sort of problem that even drives engineers crazy...
"The background during quieter parts of a track such as when theres soft vocals and instruments playing."
That probably rules out accoustical coupling such as vibration to a USB connector.
If its a RF problem, thats the nastiest kind there is to troubleshoot.
Once I saw a laptop power cord that was too close to a mousepad that would move the cursor...
Separate cables. Disable one channel. Who knows...
There is no such thing as "shielding" except with respect to circuits inside the shield. Ground or stray currents can run on shields to bad places.
Ive seen grounding make RF noise worse.
Without building up static charges on your fingers, touch anything you can find, all the units, lay hands over cables.. might find a coincidence.
My laptop, on a spectrum analyser in SW, shows a heartbeat at 25 Hz about a one second rate, seems to coincide with the HDD activity light.
"Is it possible that one of your speakers is blown?"
If not, its not loud enough...!!!!!!!
Torn cone, loose wire from the motor coil to the cone , cone separated from the frame, fix any of those with glue.
Push the cone with your fingers and see if the wire that connects to the back of the cone is loose where it was glued on.