Good advice from the salesman by the way.

I think the problem comes from the receiver's sub output. Do you have another line output that you can try from the receiver like a pre/out for the left or right channel? If you do, first turn off the sub in the speakers menu and then plug the sub in the other output to see if the problem still exist. If it does, the problem maybe in the sub, if not, it's the subwoofer output of the receiver.