How have you wired the Polk's currently? Have you just wired the high level speaker terminals or did you include and RCA cable to the Subwoofer in? Did you remove the connecting bars between the speaker terminals, as that seems a requirement for doing both.
Unless it is a really big room or you were looking for really loud then the Onkyo is probably fine. Surround speakers are used a lot less that the main LR or Centre