Powering nine home speakers

Hi. I have a large open living room and want to increase the wattage I have running my speakers. I currently have an Onkyo 7-channel amp and took the front left and right outs and have them going to an OSD Audio ATM7 7-zone speaker selector. However the Onkyo amp is not powerful enough to drive all 9 speakers strong enough. It sounds very weak. 

The only device I want to connect is a sonos connect that I have. Currently I connect the optical out of that to the Onkyo amp. Should I introduce a second amp? How exactly would I connect them? Would I run the sonos connect into one amp and take the preamp outs to the other amp? Can you have a standard front left and right out AND a preamp out at the same time?

i only want/need stereo sound not surround. 

Thanks!