All of a sudden, my twin SVS Ultras have begun to make a loud popping noise (they feel like they are bottoming out) everytime I turn the source selector knob on my Denon 4800 receiver. They even popped a few times when the knob wasn't being turned. Nothing is coming out of the center and mains or surrounds. The popping noise is at the same loudness level no matter what the volume level is on the 4800. I have the Ultras hooked up to a Sampson 1000 amp through an Art 351 equalizer. I originally suspected the ART was the culprit since it normally puts out a small pop when I turn it on. No pops are heard with the Smapson off, but the pop still occurs when ART is off and the Sampson on. Can someone help me troubleshoot this; I am afraid of damaging my two boomers. Many thanks.