The M80s will push as much air as you ask them to. You're in control of the volume. 12' x 12' is a bit on the smaller side of ideal for the M80s -- the cabinets are deeper than most people are prepared for when they see them for the first time -- but it's definitely workable.
The TX-SR606 doesn't officially support speakers with a nominal ohm rating of less than 6, so whether or not it will work well with your M80s (4-Ohm rated) is unknown. Since the room is small and the M80s are an easy load, chances are you'll be fine. Once you move into a larger space, I think you'd start to trigger the protection circuitry.
