bass distortion - dirty pots?

I have an Onkyo TX-8211 Receiver I picked up off Craig's List for my shop. The guy had it hooked up to a couple of small computer speakers and now that I have it home and hooked up to some decent Bose 301 series 2 speakers I am hearing a lot of distortion. If anyone has some suggestions as to how to problem solve this I would appreciate your thoughts.

Thank You.
I have the same speakers driven by a Harman Kardon receiver rated at 80 wpc, which I think is more powerful than your Onkyo. Anyway, while I don't really have any distortion, it is a difficult speaker to drive even though the specs for the speaker say it can be driven from 10w to 150w.