Nick, yes a vertical M22 makes a fine center channel speaker. The only sense in which some speakers are "designed specifically" for center use is the the one Fred has mentioned: the horizontal form factor meets the aesthetic or space requirements of many(not me). A vertical speaker can have an inherently wider and smoother horizontal dispersion.

I didn't catch what led to the "have no problem eq'ing multiple center channels" comment, but there's only one center channel. If more than one speaker is used in the center channel any calibration or room eq settings have to go through the one channel to the speakers identically. If the speakers are the same model and positioned similarly in relation to the listener, this works quite well.

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Enjoy the music, not the equipment.