I own the PSB Subzero i. (their smallest sub) I live in a condo unit and the I am quite content with the amount of bass that I hear/feel. However, whenever there are extended sequences of strong, low bass, I hear a funny vibration from the sub. This vibration can best be described as pulses of bass coming from the sub when there normally would be continuous bass. During such sequences, I can feel LOTS of air being pushed from the anterior port. It seems the vibration maybe coninciding with the visible "shaking" of the woofer itself. Does anyone know what this represents? Is this vibration from loose components in the unit? Is this simply my sub not able to handle the really strong extended bass from some soundtracks? Is this the sub "bottoming out"?