You guys always want to spend as much of everyone else's money as possible.
For a MUSIC-ONLY system, I think a relevant question might be "what kind of music?" If the answer doesn't involve much content below 25-30Hz, then the EP350 could be more than sufficient. If you're willing to spend the extra money and the size of the enclosure doesn't matter, I would definitely get the EP500 rather than the EP400.
I can't tell the difference between My M80s alone and the M80s with my EP350 for most music. The sub certainly helps for the pipe organ.
