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
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).
Have a pair of monos for sale if interested?
emotiva brand. 

Special amps, special instructions.
contac if interested.
500W@8
1000W@4
Actually it looks to me like the ost unit accommodates 2 amps 

OSD Audio ATM7 Automatic 7-Zone Speaker Selector w/ IR Remote Control IR Code https://www.amazon.com/dp/B004Y8MH3M/ref=cm_sw_r_cp_api_9XJ9BbP5GNFWK

it references:
impedance protection circuitry
and
Impedance-Matching Autoformers safely distribute maximum power throughout your system.

I suggest you try this first:
Onkyo front left and right speaker outs --->  speaker selector with impedance matching ----> speakers 
Imo, when you parallel all speakers without impedance matching, it give a very low impedance for the Onkyo to handle!