Polk Micro Subs are good but for the price you could choose a SVS or HSU sub instead which will give you more bang for the buck.
Another speaker brand which you could look at is PSB and there you could run a Synchrony Two setup through this AVR. IF you choose PSB then the Speakers you should audition are:
PSB Synchrony Two Fronts, Synchrony Two Center and Synchrony S Surrounds with either a PSB Subseries 300 Subwoofer or a SVS / HSU Sub.
Another speaker set which will pair well with Onkyo will be PSB Imagine based setup.