You don't mention what sub you have, but I suspect there may be corrosion in the sub's volume pot. If you can access it, try spraying some electrical contact cleaner, while you twist the pot up and down. This may clean things up, and get your sub working again.
Also, if that doesn't help, see if the sub's volume control can be bypassed, and control its output through your preamp-processor (assuming you have one).
Good luck,
Dan
Also, if that doesn't help, see if the sub's volume control can be bypassed, and control its output through your preamp-processor (assuming you have one).
Good luck,
Dan