I have two listening rooms in upstairs rooms (in my home & cottage), but neither is as large as yours. Never the less, I've never had much luck using panels of any kind in those rooms. I just don't think dipoles lend themselves to angled side walls. I'd try shifting to a good dynamic speaker, not because of peak output, or sensitivity, but because I think you will have a much easier time getting integration with the room. Also, assuming it could be done, I'd consider setting up a near field listening set up turned sideways, with the speakers against one of the sloped walls, and you sitting against the other. This obviously wouldn't work with the tall Maggies, but a pair of short towers, or mini-monitors could be just the ticket set up that way! I wouldn't think in terms of "filling the space" so much as using the space to keep the room from getting in the way of the speaker/ear interface.
Cliff
Cliff