The M40s and M50s are very similar brethren. Same driver/tweeter type, etc. but the M40s were the smaller version of the M50s with a single woofer instead of two, and hence a slightly smaller tower.
Both are 2-way designs in contrast to the M60 3-way driver design which makes a huge difference.

I did a blind testing of the M40 vs. M60 awhile back (these would be the M60 originals, not the newer version 3).

The link to my humble thoughts (and photos) can be found here.

Another review of the M40s.
http://www.soundstage.com/revequip/axiom_millenniam40ti.htm


Incidentally, Axiom needs to update their vintage info page to include recently defunct models.
http://www.axiomaudio.com/vintage.html

"Those who preach the myths of audio are ignorant of truth."