The only reason is the placement of the main speakers. For example, until I piled up a whole bunch of boxes of paperback books in a corner of my room, even setting the M80 at 80hz was problematic (terrible booms). Now that I lessened one of my "corner problems", I can go down to 50hz with only a small range of boomy notes (noticeable mainly with pipe organ) near 40hz. Setting the xover at 60hz provides a very good bass distribution accross the whole room, better than at 80hz. A lot of these choices depends on placement.
The M80s alone have really good bass. Before I received by new HT unit, which forced me to put the M80s much closer to corners, the bass was astonishing, even for the 2 months I waited to buy my EP500.
