@mijostyn- There are a couple different types of foam surrounds, that have been around for a long time. I’ve seen some, that have survived from the 70’s, without deteriorating. Temps and humidity can/will have differing effects, as well. Whenever I replaced a rotted surround(lots of ’em, in Florida), I asked whether the customer wanted to insure, never to having to do it again, with a butyl compliance. It’s not unheard of, for an entire surround to fail (tear around the circumference) at once(ie: without prior visual indicators), with one good excursion. At that point, the voice coil is in danger of being damaged/destroyed, by contacting the magnet gap(probably what happened to your sub) . That won’t happen, if the surround isn’t already rotten, beyond a certain degree. Guess the best test, for an invisibly rotted surround?
