I figured it was a ported cab. When you get it reconed, add a large capacitor (you'll have to know what the cab's tuning frequency is and amount of power being fed to it to calculate its value) to the crossover to act as a high-pass filter and that should keep it from happening any more. People who use eq filters ahead of the power section don't realize that amplfier operation can produce sub and infrasonic power on its own. When you put the cap in there, anything below the r/c curve never gets to hit the speaker and is safely shunted to ground. The end result is that your speakers will last longer, be louder and more efficent (sub-tuning power dramatically affects resonant frequency response) and tighter sounding.