it all depends whether the monitors have any tone control built in, and if you choose to use the 7000 parametric equaliser to tailor the sound.

For example if the left/right monitors are rather bright and have no tone control they can be connected to normal line output and used full range, and if the 3rd speaker is more suitable for bass duties it can be connected to the sub out. The sub out can then be configured for bass, bass and drums, or any of the other part accompaniment selections available and the hall high/low sound balance achieved with just the volume controls on the 3 active speakers.