Hi, I've noticed recently that my subwoofer no longer vibrates the way it used to. I still get low frequency sound coming from it, but it used to vibrate a lot when guns go off, explosions etc. I have tried various different dial settings on the back, reset/auto configured my amplifier, but nothing seems to make it work as well as it used to. Any ideas please. (could something have vibrated loose inside?) All assistance appreciated. Thanks and Regards Mike