Tom,
I have to echo gem's reply to you.
The difference b/w the M60/M22 and M40/M50 is mostly in cabinet size, driver numbers and for sound, the upper end as was mentioned.

Here are some links to reviews (if you have not already seen them) with some of these speakers which may help you somewhat (or maybe not):

See the bottom of this post for a link to 3 reviews including M40s, M50s and M22s.
This link may also be useful.

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