I've only heard the on-wall M22s -- I have them in my own living room as part of a 5.1 system -- but I hear that they sound more or less identical except for bass range. The product specs indicate that the in/on-wall M22s play 10Hz lower than the on-wall M22s. You didn't mention whether or not you'll be using a subwoofer with this setup. If you will be using one, the bass extension difference is inconsequential and you should just go with the On-wall M22s.
That being said, you may like the Axioms so much that you'll
want to have more around the house and buy another pair in two years anyway.
