2P, the default crossover setting on your 788 is a uniform 80Hz setting for all speakers. However, there is provision for setting separate crossover frequencies for the different speaker locations. Despite this, you might find that staying with the uniform 80Hz is best. Rather than wasting the M80, you might view not using 80Hz as failing to take full advantage of the fine sub which you've described.

-----------------------------------

Enjoy the music, not the equipment.