HELP! Surround speaker expertise needed:

Question

I'm a newbie @ home theatre systems. I'm 99% certain my connections (all monster cable) are correct. I'm running: Denon AVR-1803 receive w/optical to my DVD Polk Tower fronts Cerwin-Vega E-75C Center Polk sub Polk bookshelf surrounds. Problem is this: Everything sounds great except that I'm getting distortion from the surround speakers. As everything is new (or almost new), I'm at a loss! Anyone have any suggestions??