I don't have a subwoofer pre out on my old receiver and want to hook up an Onkyo SK10W subwoofer. The only speaker connections on the back of the subwoofer are for high level input which I assume is for the L & R speaker wire hook ups from the receiver. However, there aren't any output hook ups to go from the subwoofer to the speakers..the only other connections are one set of RCA R&L inputs and one set of RCA R&L outputs....do I bi-wire it from the receiver to the sub and to the speakers from the receiver or am I out of luck???

SOURCE: Connecting subwoofer to receiver.

have u checked that the sub option on the reciver is on?

SOURCE: Onkyo Subwoofer Pre Out Problem

May not necessarily be the out put, discontect the sub from the receiver and see if it still makes a hum, Making sure that you turn auto on, off on the sub woffer.
If it still makes a hum, then you have a earthing problem, you can get a earth filter put on your internal wiring in your home, or you can remove the earth pin on your sub woffer

SOURCE: subwoofer connection

if this is ao powered sub, that plugs into the wall, there should be no problems, if it is powered solely by the reciever it may work fine but be sure you dont exceed amperage on the subwoofer.

SOURCE: how to connect a kenwood powered subwoofer to a 7.1 onkyo receiver

Easy.Get a 2 into 1 rca lead and plug the single end into the receiver and the other 2 into the amp

SOURCE: Onkyo tx sr503 No

Turned out to be a level problem.