A pair of M80s is as good as a pair of EP400's, IMO, and during my brief time with the Ep400 I found its sound became lost when the EP600 was playing. I am not sure if adjusting the crossover on the 600 would have helped the 400 become more pronounced by keeping the 600 out of the range the 400 would be playing, hmmmmm, interesting............
With M80's doing the bass duties below 80Hz you run into the problem of best placement for bass versus placing the M80's as the main speakers. If you have the ep400 you don't have this problem. With two ep400's placed at the midpoints of the walls facing each other the bass would be uniformly flat in every seat.
