Dermott, welcome. The M2s should work very well when crossed over at 80Hz with a STF-2. If the sub wasn't used, then certainly the M3s across the front would be a better choice, but there's no good reason not to use the sub in all circumstances. Keep in mind that the room doesn't necessarily have to be "filled"; the requirement is simply to have an adequate sound level at your listening position.

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Enjoy the music, not the equipment.