Mike, welcome. Yes a third M2 would be a good idea for a center speaker(you might consider first trying the mains handling the center channel by way of the "phantom" effect), but not tipped over on its side. For wider and smoother horizontal dispersion these speakers have to be vertical, as they were designed to be used. If you can't figure out some way to position them correctly and you decide to use a center speaker, you might as well use a horizontal unit that fits the space.

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Enjoy the music, not the equipment.