If using a stereo cinch/cinch use either the red side or the black side, no importance it's the same connector !

Use a mono cinch/cinch (sometimes called subwoofer cable). You can use any usual cinch/cinch but a good quality one is better. Plug it in in the the amp right output (check your Onkyo manual) and in the LEFT/LFE input of the sub. Cheers !

Cinch to cinch