how to connect a passive sub to my av reciever

If your Onkyo has a "B" set of speakers posts then connect the sub to the "B" binding posts. That will send the full range to the sub and set the passive sub's crossover manually as you will get the full range from there. If you have zone2 it is the same as B set of speakers.

When you say passive sub, I believe you mean unpowered. I would say connect your sub through a dedicated 1-channel sub amp like an NHT MA-1. I have 2 for sale.

One idea;

Sub out (via RCA or XLR) from pre-pro or AVR ---->(RCA or XLR input on) the Subwoofer Amp, then Subwoofer amp out (via speaker cable) ----> (via speaker cable) Passive Subwoofer in.