If you're running a 7.1 setup, it is my opinion that QS series speakers should be on the sides, and either QS or
direct radiating speakers (M3, M2, M22) in the rear.
In a 5.1 configuration, you are correct that you could use either direct radiating speakers (M2, M3, M22), or QS series speakers a your surrounds. I personally prefer the QS series speakers as surrounds, but there are others here who prefer direct radiating speakers.
Here's what the Axiom FAQ says about surrounds.
"
Ideally, surround speakers should be either wall-mounted or stand-mounted to either side of the couch or primary listening area, from one to six feet higher than seated listeners' ears, and slightly forward or rearwards of the listening area if you wish. Axiom's Quadpolar surrounds are especially versatile and can yield excellent performance with stand or wall-mounting to the rear as well as at side locations. (see our dedicated stands for the QS Surround speakers in the Store). Rear center speakers for 6.1 and 7.1 installations are intended to go on the rear wall at about the same height as the surround speakers."
Since your couch will be flush against the back wall, I would think 5.1 would do just fine with the QS series, or direct radiating speakers, mounted on the side slightly in front of the listening area. In your other post you mention not having walls on either side of the couch, so you would have to mount the direct radiating speakers on stands of some sort. If you went with the QS series speakers for surrounds, you could mount them on the
Axiom stands, or hang them from the ceiling (if that's possible) with either the
Omnimount ceiling mount, or the
Axiom Full Metal Bracket (FMB).
