The best way to hook up a subwoofer to an amplifier with no dedicated subwoofer out is to use a "Y" splitter cable.
- Take out the pre-out / main in jumpers on the rear of your amplifier.
- Replace the jumpers with "Y" connectors. (The male side coming out of the "pre-out")
- Connect one of the female sides back into the "main-in", while connecting the other side of the "Y" to the subwoofer. (RCA cable extentions will be needed as well in most cases.)