Powering nine home speakers

     Okay. 

     You can connect the left and right front outputs on your receiver to a separate multi-channel amp.  This allows you to connect some speakers to the speaker outputs on the Onkyo receiver and the remaining to the multi-ch amp's speaker outputs. Whatever source you select on the receiver will play through all speakers.

     You didn't state your budget and there are many types of multi-ch amps with a wide variety of channels (3 channels plus), a wide variety of watts per channel and a wide range of prices.  I'd recommend matching the watts per ch on your multi-ch amp to the watts per ch on your receiver.  If all your speakers are all the same brand and model, this means they'll all have the same efficiency rating (listed as decibels at 1 watt).   This means each speaker will be producing the same volume output at any setting your receiver's volume knob is set at.  

     Just google "multi-channel home audio amps" if you'd like to see the various amps available.
Tim

     

 

Trade in the Onkyo and get this.
https://www.mcintoshlabs.com/products/amplifiers/MC8207

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??

No movies. Just music. What is a pro/distribution set up?

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