The concern I would have with using M50s for rears is that they are not timbre matched with the M60s. The M50s are reportedly (since I don't own them) warmer and more "laid back" depending on your perception of these great, and vague terms we all use for describing sound.
If you are going to listen to music, then a directional speaker (like the M60s and M50s) are desireable, however, if it is for movies, you REALLY want the QS8s. They are just AWESOME for surround sound in movies! You can find information all over the internet about types of speakers to use for surround speakers in a move viewing environment. They will all tell you to "difuse" the sound field. Any Axiom speaker besides the QS4 and QS8 do NOT difuse sound.
You are in a bind, my friend... You want awesome sound, but still have to live with your wife. My wife was a little freaked out when I unboxed my SVS PCi 20-39 subwoofer (39" tall, and about 16" around cylender shape)...
Every time I hook it up (since I am still working on finishing my basement, and the speakers sit in a corner in our office - what a waste of good sound) my wafe makes some comment about the "monster sub." I have a spot for it in our basement that will help to make it less noticeable, but trust me, we all (guys anyway) have struggled at some point with the WAF (Wife Approval Factor).
Please do not take my comments above about the M50s as "holy crap, I need to get things changed" but I would sure consider it. The QS8s are fairly small, and are VERY versitile in regards to placement of them.
Just don't want you to be unhappy with your set up later on.
