I read that one of the forumites has a bass unit that's started to 'flap' at normal listening volumes after 15 years and he's thinking of replacing it. I can't find the entry to respond directly so I'm hoping he or she will read this.
I had a similar experience some years ago with my Wharfedale 12inch woofers in a home built cabinet. I assumed they were gonners and gave them to my brother-in-law and then I went and bought some expensive replacements.
My bro-in-law put the Wharfedales into his own cabinets and they were perfect. It seems that, over the years, the connections between my amp and the wharfedale speakers had slowly gone hi-resistance and this allowed them to flap. By replacing them I had inadvertantly tightened up all the connections and brought resistance back to normal.
It was too late for me to revert so I let him keep the speakers and wrote it off to experience.
So I suggest that, before consigning your sub-woofer to the scrap heap, first check all the connections between amp and cabinet, and then INSIDE the cabinet. Could save you some heartache and dough.