Will your Onkyo support unique sources for each zone? If not, you'll be limited to having to use the same audio source for all zones all the time. I often find it nice to be able to have each zone play something different.
30 watts per channel will be more than sufficient for the Polks, so you could look at any of the multichannel amps available. The common configuration for this is a 12 channel, 30 watts per channel amp.
Most of these type amps will allow you to bridge two channels for higher output. Since you only have 8 channels, you could bridge a couple for any zone you need/want to boost the volume…
Audiosource, Niles, OSD, Dayton, Sonance all have options available.
I would highly suggest investing in a source system other than your Onkyo… I have a Denon that I tried using, and found myself getting frustrated with always having to go into the wiring closet and muck with it to get it setup right. I now use Apple Airport Express units as a source for each zone… airplay from any iOS device, super easy, wife approved.
Of course, this is something you could always upgrade to later once you have a nice amp… so you can start with your system and improve it if it become unwieldy.