Dermott, there have been lots of comparisons between the M3's and M22's over the years. I think I've even done one myself. You might want to run a search.
Anyway, the long and the short of my view of both, is that they're both awesome speakers. I own both the M3's and M22's. I use the M3's in my bedroom and M22's in my main system. I have done some direct comparisons between the two (but not that extensive).
I found that only in comparison to the M22's, did the M3's "miss" a little of the midrange clarity, and forward sound of the M22's. The M3's do have better bass, and even though I have an SVS sub in my main system, I have contemplated swapping the M3's for the M22's for that reason. I decide against it, though, due to the great combo of the M22's and a good sub.
Bottom line, for a bedroom, I think the M3's are perfect, especially if you won't be using a sub. I have the benefit of having built-ins in my bedroom, so I am able to keep a small sub (Velodyne VX-10) out of sight (helps with WAF) within a cabinet in my bedroom. Obviously this is not audiophile quality, but the bass does reinforce the M3's. If you will be using a sub, the M22's would probably be overkill in a bedroom and you could get away with M2's.
