Please do not purchase this cable for this purpose, as it is not a combiner but is intended for connection to devices with an unbalanced stereo input.
If you connect it to an active speaker with a balanced input, you will not only lose the balance, but you will also only get a differential signal from the L and R!
However, if you have one of the 6 outmixes available, you could use it (Post Fader, Post Comp, and all faders at 0 dB) and get a mono signal at the corresponding output.