You have a couple of options. If you buy a pair of M22ti's to replace your mains, there will be a considerable change in sound. The M22ti's are more detailed and "brighter" in the highs. (I personally prefer that sound, but many people like the more "laid back" sound of the M3's.)
Or like you also mentioned, if you keep the M3's up front, because you want to retain that same "laid back" M3 sound, you could get a couple of QS's for surrounds. This will disperse your surround effects more, and give you a more enveloping sound, much less direct sounding then the M3's you are using as surrounds now.
Since you say you use your setup for 80% TV/Movies, I'd opt to keep the M3's up front and upgrade your surrounds to a pair of QS4's.
