The only bits I could see in the manual are section 8 9 and 10 on Page 6, just below the picture of the connectors.

Regrettably, although it would be a nice feature, the Centre out is not acting like a mono sum of the Left and Right channels; if it did the manual would make a big point of it.

It is a great pity that the manual does not contain a block diagram of the circuit, especially given that Behringer have such diagrams for mixers, crossobers, compressors.....

I think KingFrogs idea is pretty good, best of luck.