Are you using active high pass on the monitors, or are you running them full range, with the sub simply augmenting the lowest octaves?
If you're not running an active XO between sub and mains, adding the ability to cut everything below 80Hz from the mains will allow them to play with more dynamic range for two reasons:
Reduced excursion requirements on the mains allowing cleaner midrange reproduction
Reduced peak to average ratio on the amplifier, allowing better dynamic peaks for more realistic impact.
If you're not running an active XO between sub and mains, adding the ability to cut everything below 80Hz from the mains will allow them to play with more dynamic range for two reasons:
Reduced excursion requirements on the mains allowing cleaner midrange reproduction
Reduced peak to average ratio on the amplifier, allowing better dynamic peaks for more realistic impact.