Andrew, you didn't specifically show your planned listening position, but the length of the room indicates that you'll likely have enough room behind you(at least 4')for a good rear sound field to develop. Especially since you already have the M2s, they seem to be a good choice for the back surrounds since there's likely to be enough room(at least 6')to the listening position for the sound to be well spread out.

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Enjoy the music, not the equipment.