Hooking up a REL sub using a 2-cored cable

I just bought a second hand REL Stadium 2. However, I wasn't paying attention when the guy handed me the cable with Neutrik connector. It has only two cores. How do I connect my REL to my stereo amplifier (Marantz PM 17)?

In the REL manual it only addresses a 3-core Neutrik cable.

The guy I bought it from demonstrated the sub working with this cable, so it IS possible to get it to work! :)

Thanks.