Dougie, the greatest difference would likely be in the lowest bass, where the M100s, if playing alone, would have the capability to play louder. Since you're using the EP500 to play these frequencies, however, there might be only a very subtle difference between the two(even on those Stravinsky items we discussed a couple years ago).
My thought would be that rather than new main speakers, a second sub, properly positioned to minimize the effect of room modes, would be a more significant addition.
