I have a set of Altec ADA70 speakers and whenever I tun the speakers on the sub emits an audible "thump". Taking off the grill reveals that the woofer is being sucked in and not released when the speakers are turned on. This in turn limits the bass output because the the movement of the woofer is restriced. Also, the speakers start to distort after being on for a hour or so at moderately low listening levels. Does anyone have any ideas about what might be causing this and can it be easily fixed. I would RMA them but they are out of warranty.