I think this should work, I did something similar a long time ago.
I ran the sub out of the receiver I was using to one of the inputs on the old receiver, let's just say it was the CD input. I then hooked up one of the speaker outputs to the passive sub. Worked perfectly.
3 - JTR 228's LCR (game room)
4 - DIY Sound Group V-8 Coaxials in slanted boxes (game room)
4 - PSA Triax's (game room)
3 - DIY Sound Group V-8 Coaxials in Ported Boxes for LCR's
2 - SVS SB13+'s (living room)
1 - SVS SB12-NSD (bedroom)