Troy, two EP-500s should certainly do very well. One suggestion would be to make the placement exactly at the midpoints of the opposite side walls(i.e. 9' from the front), which is the point where the room mode for the 18' dimension is at a minimum.
Yes, setting the crossover at 80Hz with that setup should work well(but with the M80s set "small"). The idea is to set it high enough to take a significant amount of the low bass load off the mains, yet not so high that the subs are localizable; generally 80Hz does that.
