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!
jj91709
Well if this is a stereo set up, then your answer is to do more of a pro/distribution type setup.

If you are using this for movies, why oh why do you have 9 speakers in your ceiling??
The OSD Audio ATM7 7-zone speaker selector dose not provide impedance matching, try one speaker distribution selector with impedance matching, if still has insufficient music volume then add a higher power stereo amp connect to the Onkyo front R/L pre-out to drive those speakers.
https://www.amazon.com/5-Zone-Stereo-Speaker-Distribution-Controller/dp/B01G7TW8W0
Onkyo R/L pre-out ---> stereo power amp ---> speaker selector with impedance matching ----> speakers
Thanks imhifiman and Erik.

In the case of the speaker distribution controller , in this arrangement would I get rid of the ost audio unit? And would I simultaneously run front I/r from the onkyo speakers to the controller and the pre-amp outs out from those same channels? I just can’t remember if those are mutually exclusive (you can do either pre-amp out or speaker level out for front l/r).