The cable for the sub is just a coaxial cable with no ends, so I need to stick an RCA end on, ... You might consider terminating the coax with a BNC female, then adapting to RCA male.

I'm getting no sound from my subwoofer working when using my pre-wired connections through the wall, though it works fine if I connect directly. Here's what I'm doing - I'm using an RCA cable to ...

But the box only comes with a 1/8th inch aux cable ... $15 for either the 1/8" to RCA or 1/8" to optical adapter, neither of which is very long (though they don't need to be).

The connector is a standard, circular RCA connector - the kind that’s found at either end of a pair of analogue audio cables (or 'interconnects'). But don’t be tempted to try and use a standard RCA ...