My room size is an average full basement. Watch movies more than music, but rather it sound better for music, if that makes sense.My question is, should I go for the M50's, and throw the 40's in the rear? Or go with a bookshelf like the 22 and keep my 40s in front? And should I factor in that my 40s are kinda short, and the bookshelves on stands would be taller? Heard thats a no-no.

Sorry for rambling....Thanks for any help!

I suggest you check out the QSx series for rear surrounds.
If you are set on using full size direct radiating speakers, take a look at a personal review i did earlier this year with my impressions of the comparison:
the link
Hope that helps.

I would get the QS4's or QS8's