To connect a passive mixer to a power amp would I need to go thru the Main Out on the mixer to the power amp or somewhere else?
Yes, the Main Out, the level of which will be controlled by the Main or "Program" sliders.
You can take the main outs of your mixer (XLR) connector to the main input (XLR) connector of your power amp... Some people also route the mix to a sub out (usually 1/4 inch plugs) and route that to the Stage monitors so you can control their volume separately. Always make sure the power is OFF and the volumes is turned to minimum when making connections.