I have a Behringer Xenyx QX1202X mixer and wanted to have a monitor attached. So I bought a single M-Audio BX5 active monitor speaker as I only need to monitor in mono. I have connected the 2 Main Out connections to the monitor speaker using a cable  

Sweex XLR Male to 2 x Phono RCA Plug Stereo Audio Patch Cable Adapter Lead

The problem is that the monitor outputs a weird sound unless I pan the mixer inputs one way or another - when using the USB to provide the input then obviously I cant do that.
Can I alter the wiring on the XLR male plug to provide a mono signal to the active speaker - if so - how?
Thanks in advance