Looking at the rear of the Onkyo: http://www.jr.com/onkyo/pe/ONK_TXNR515/
It looks like you have a 'Pre Out Subwoofer' and a 'Line Out Zone 2' at the bottom, a little left of center.
Hook a set of interconnects with RCA ends from the 'Line Out Zone 2' into the power amp inputs. If that doesn't work, try the 'Pre Out Subwoofer'.
Chuck