also. check and make sure the spade lugs on the woofer are tight. these often become loose in subwoofers. removing and retensioning the grippers will usually fix this. occasionally the tinsel wires in the woofer wear out, and would cause dropouts as well as noise. the tinsel wires would look darker near the break, because the remaining wire strands heat up during operation.
because the amp is within the woofer cabinet, the most common problem would be, as already mentioned, bad solder joints.