I have a home theater setup with a receiver and subwoofer. The sub "rumbles" (sounds like an earthquake) at certain low decibels. I tested the sub to see when the offensive noise happens, and it occurs with any frequency between 15 and 30 decibels. Under 15 and over 30 and the bass is clean and clear. To fix this, is an FMOD high pass filter the right way to go? Everything I've read about FMODs or hp filters in general says to place it between the sub and the amp, but I don't have an amp in my setup. Getting to the actual speaker to pad it or anything similar is impossible based on the construction of the sub. Oh, and the model of sub is Sony SA-WMSP75. Thanks!