Bad surround may be the issue & hopefully no permanent damage has been done., There are a few surround re- foaming kits available on eBay for short $ for the original Large Advents that are worth trying & because there were so many of these speakers sold, there’s probably entire woofers available also. Jensen then distributed & I think made a replacement woofer for it that has no Masonite ring & a black metal frame that is a perfect fit & works very well. I owned a few of the Advents back in 1974 ( one pair & then a stacked pair) & again later on in the 90’s ( left in a home I bought!).
I blew a few woofers playing them at stupid high levels but overall they’re were a great speaker in its day for $210 /pair in Walnut cabinet). For the equivalent & $1200 or so today, there are speakers w/ better high end & image better but few if any w/ better bass. They are warm & musical & their faults are more omission than commission. Get them fixed & enjoy a true classic!