Reply to PA Setup Questions

Ok, i've been using a pretty simple PA system for my shows for about a year now. Just 2 Peavey PR12's and a old school powered mixer. I'm looking to add a subwoofer to it because im starting to play for bigger crowds and need the 'omph' of the bass to be felt. I'm looking to go with an unpowered mixer and a power supply since my current mixer only has one input for the speakers on each channel, and I was wondering if I would have the wiring down correctly. It would go: Speakers and Subwoofer-Mixer-Power amp. (- representing wires) I don't know much about PA's so any help would be nice. Thank you