This is what happened to my 12W4 sub after 10yrs inside the trunks of two cars
I have a grill on the box and can not see the condition of the sub until it got distorted. When I removed the grill to check the spkr...

 

Was it still playing in that condition? How long did you notice distortion before taking a look?

I am just wondering if this took place slooooooowly, or if it deteriorated to a certain point and then fell apart rapidly...

 

most likely ti deteriorated slowly at first until and then the rubber surrounds quickly tore off. It happens to most subwoofers. Still I prefer foam surrounds over rubber.

 

i noticed the distortion several yrs ago but i was too lazy to check
it played fine at mid volume also
crank it up alittle more produced an anoying cracking sound
i now switch to rockford and i love it

 

JL to this day still uses treated foam surrounds. I've heard claims that rubber surrounds last longer but haven't seen any real world test to confirm this. Considering most sub manufacturers use foam surrounds I would think using foam is better.

You can actually buy a surround repair kit to fix that. I've tried these kits twice myself but the sub never sounds the same. They pop at medium to high volumes. Having a shop repair them probably won't be worth it and cheaper just to buy a new one.

 

there has to be a way to treat the surround the same way you treat weather striping to make sure it does not dry and crack from the heat.

Silicon spray on a tissue and then gently rub it on the surround?