Jason, your last comment that there's still some hum even with the coax completely disconnected indicates that this isn't a ground loop, since with no cable connecting the sub to the other components there's no path to form a loop. If the sub hums in this situation it indicates that an internal defect in the sub amplifier has probably developed. Neither grounding yourself to drain your excessive personal magnetism nor buying a ground loop isolator should help.

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Enjoy the music, not the equipment.