A subwoofer is a mono device, and only needs a single channel of audio fed to it.
The reason our subwoofers have a twin stereo input is so they can be used with the pre-outs on older stereo amplifiers. This is because the bass for the left and right channels is sent to the subwoofer separately, and then combined inside the unit.
AV Amplifiers send out all low level bass, below a frequency cut off specified on the Amplifier, through a single RCA phono socket (often called 'sub out'). Therefore, you only need the single lead, from the rear of this amplifier, into either one of the inputs on the subwoofer.
The subwoofer will automatically detect whether it is receiving a stereo or mono signal, and adjust its output level accordingly, so that the volume from the sub remains the same.