I know Debbie told you to go 160HP but I strenuously object and recommend you go with a 180 non HP. You're in a big room, 20 ft MLP and the 160's single mid will strenuously object along with me. smile

Thinking more about it, M22s for the sides make more sense than M3s. Surround effects are all about middle to high mids and low highs where the M22s excel. They are also more sensitive.

On-wall is always better than in-ceiling for the rears.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated