14' x 14', hmm. I wonder if you could set up on a 45, play out of one corner of the room, opposite the dormer of course? Have you tried that? The back wave off the Magnepans wont have a flat wall behind it that way, but instead be bounced sideways across toward the other speaker before it's bounced back at you, increasing the delay time. You could put a diffuser in between and behind the speakers a bit to break the path in both directions.
I work at ASC where we sell TubeTraps, and the TubeTrap was originally designed to go behind Magnepans to help absorb some of the back-wave bass before it bounced off the wall back at the speaker. It also scatters the treble from the back wave, depending on how you have the treble reflector positioned. So if you stay set up like you are with the dormer behind you I'd recommend a combination of upper bass absorption and treble diffusion.