Tom,
In response to your question, the M40's have similar characteristics to the M3, which is a bit warmer sounding than the M22/M60, which are more forward sounding. Some prefer the more laid back sound of the M3/M40, while others prefer the more forward sound of the M22/M60. There's no right answer, just personal preference.
The M50 is kind of the forgotten Axiom. It is very rarely discussed, but I would suspect that for most people, if you are going to step up in a tower speaker past the M40, they figure they may as well take the leap to M60, as the M60 is so highly regarded. I am sure the M50 sounds great, but it is just not discussed frequently.
In your case, if you are deciding between the M40, M50 and M22, I would think you would be advised to forget about the M40, unless you really do not like the more forward sound of the M22's. As between the M22 and M50, I can't really speak to that as I have never heard the M50's.
In response to the earlier question on whether the M22's will fill that size room, I use the M22's in a 12'X 21' living room, which opens in the other direction into an 8' X 10' foyer area, and the M22's fill it up just fine. There are no issues with the M22's not being able to handle my room.
