Crash, the ideal is to have a center speaker identical to, or at least as similar as possible to, the main speakers. An M2 is essentially the upper 2/3rds of the M22 and several users have reported here very good results using it as a center. A vertical center in general(the unique configuration of the VP150 apparently is a significant improvement) has better horizontal dispersion than a horizontal center. It of course can also be placed right on top of the TV.

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Enjoy the music, not the equipment.