I have a Onkyo M-501 amp. I want to drive a JBL sub that has dual inputs. My problem is that my amp will only push one channel at a time because I don't have the Onkyo preamp. There is an input cable from the preamp to the amp which enables both channels. This seems like a marketing gimmick by Onkyo to get you to buy thier preamp. My question is how to disable the plug and be able to use both channels at once. Thanks in advance!