I have Acoustics Research AR-1 tower speakers with built in Bob Carver 500 watt Sunfire amplifier powering 15 inch subwoofers. Have developed a gargling sound in one subwoofer in low frequencies above low volume. Switched speaker cables to verify problem in speaker and not in main amp. Turned off Sunfire Subwoofer amp (there's a switch on back of speaker) and no distortion. The Subwoofer amp works without distortion on low volumes but starts gargling distortion at higher volumes. Any suggestions welcomed.