If you like the sound of the M22s but just want "more sound" most people would recommend the M60s over the M50s.
There are sort-of two families in the Axiom lineup -- the M2 / M22 / M60 / M80 family using 5-1/2" drivers to cover the midrange, and the M3 / M40 / M50 family which use a 6-1/2" driver to cover the mids.
Don't know if you can say that one family sounds "better" than the other, but from reading the reviews and looking at the frequency response charts the M3/40/50 tend to be a bit higher in the midbass (100 hz) and a bit lower in the midrange (2000 hz) than the M2/22/60/80.
Only talking small differences here but enough that people who compare the speakers side-by-side do comment on the differences. What kind of music do you listen to ? If I had to REALLY generalize I would say people seem to like M3 / M40 / M50 better if they listen to a lot of kick-ass rock, and like M2 / M22 / M60 / M80 better if there is also a lot of classical or progressive rock in their library. Jazz seems to sound great on both.
Here's a link to a comparison between M22 and M50. You won't be too far wrong if you think of the M60s having all of the "big open sound" advantages of the M50 with the characteristic sound of the M22 and probably even better deep bass.
http://www.axiomaudio.com/boards/showflat.php?Cat=&Board=ST&Number=47919&page=&view=&sb=&o=&vc=1
I have not seen a comparison between M50 and M60 yet.
Bottom line, I think, is that if you are happy with the M22 sound and would like the same sound but "bigger" (more open, deeper bass) go with the 60s -- if you find the M22s too bright or too forward then you'll probably prefer the M50s.
Again, as I understand it the differences are not all that large.
