My cabinets, a 4X10 and a 1X15 make a strange farting clacking type of sound when I slap the low B-string. This only happens on these particular cabinets, not my others. I am running a SWR Bass 750 into them. I guess I'm getting 375 watts per cabinet? Does anybody have any idea what it could be. I tried lowering the pickups, it didn't help. Am I clipping the amp or underpowering the cabs? Any help would be appreciated!