I could be wrong, but I think normally when you use Zone 2 on a receiver, they share the surround back channels, and my your home theater a 5.1 setup when being used for zone 2. I don't think you can use the front channels, could be wrong though.
I would think your Onkyo should give you plenty of power to drive the m22's, how loud do you listen? Even if you get an amp with double the wattage, that will only give you another 3dB's, not much.
Do you have plenty of ventilation "around" the receiver? Onkyo's run hotter than some other receivers by design. How hot is it running when you add in the center. Also, have you changed to Ohm switch/setting to something different than 8ohm?
