M3s would be better than nothing, for sure. You can always try them as surrounds and see if you find them satisfactory. I haven't used bookshelf speakers as surrounds for several years, but I remember when I first got the QS8s and being really impressed with how much more the sound wrapped around me. I vote for QS8s as surrounds if money's not too much of an issue, but like I said, you could certainly try the M3s first.