I don't think the problem is with the cable. It seems like it is the connector inside of the subwoofer. Wiggling the cable doesn't make any difference. Only putting a little pressure on the entire plug so that it pulls the connector over does it make the sound work.
Maybe just resoldering the plug would help.
I wish companies would think a little about how far some of these plugs extend from the back. I've had similar problems with connectors / plugs on computers, speakers, etc. If they could use a right-angled plug or recess the plug it would help.
David.