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! :)

Why not just call the guy and ask him how he did it?
Buy a cable with three conductors as it designed.
I would call REL. They will help you. I only use 2 cables on my setup but I run a bridged amp using a differential setup.
The guy you bought it from was likely getting the signal from one channel of the amp. I love Speakon plugs (use a lot of them in pro audio) and recommend the angled ones if the REL is near a wall, and since Neutrik plugs allow for multiple wires it seems silly to be bothering with a 2 line solution. I use 3 parts of a nicely made 4 conductor spiraled (rf rejection) cable that works very well.