About the only thing that comes to mind for me is that it could be the driver's internal speaker leads from the voice coil that are soldered to the driver's input connectors (or binding posts, if equipped). Generally as woofers age these bare, braided wire leads can, over the years, get looser, more flexible and flop around a bit more during excursion than when new. The leads themselves should be designed to be long enough to allow for maximum excursion and also short enough to prevent slapping against the back of the cone (or something else back there). It doesn't happen all the time, but I know it has come up once or twice before in regard to heavy excursion...in your case, think very low bass response at higher SPL...as in, as you say, HT and gaming, maybe?? A possibility anyway. Maybe the best way to confirm that would be visually somehow, but I can't think of a foolproof way at the moment to do so...although you could try removing the driver temporarily with it still hooked up and play some of the offending bass through it and look behind the cone in action to see if it's a possibility, even though the woofer will no doubt behave at least somewhat differently when unloaded from the box. Best of luck, whatever the cause.
