[Funkmonkey]

Assuming you've eliminated the cable as the problem, and by the sound of it the sub is not the problem since you tried it on a different system. I would open up the STRDE597 (warranty is done already) and visually check for any loose connection along the sub out that is giving you trouble. If everything is gravy, (if your afraid of electricity stop here and take it in or get a new one) I would power it up and play some music with only the sub connected and then start giggling the wires on the sub out to see if it cuts out. If it does you found your problem, if not take it in or buy a new one. and make sure to dispose of the old one in a "green" way.