craig- i have no 'official' reason, but it would appear they just wanted to make a little beefier bookshelf speaker. when looking at the specs, it seems as if they were just trying to go with a little bigger woofer. the MZero and M1's have 3.5" and 4.5" drivers.. while the M2 and M3 have 5.25" and 6.5" drivers. the tweeter has remained the same. they all have the axiom 'wedge' design, and all are ported. although, i did notice the ports on the M1 are smooth, and dont have the 'gorilla' grooves that we all love to touch and feel.
anyway, i have ordered some M1's, and they should be here next week. i will be sure and let you know how they sound.
BTW- i managed to get them for $130 a pair at canadianhifi.com. i thought that was a great deal.
bigjohn
