I own a Polk Audio PSW12 Subwoofer. It is connected to an Onkyo TXSR600 Receiver. Quite often when i have the volume turned up loud the sub will pop or crackle. the Sub has adjustments for Low-Pass Crossover, Phase Switch and Subwoofer Volume Control. I have no idea what these should be set to for optimum sound and performance. Can somebody help me?