I had a pair of Kef 107s that I bought just after they were released - mid 80's. Every bit of foam and rubber insulation on them had decayed. These speakers had a pair of woofers completely enclosed in the cabinet, in an unusual configuration where they were bolted together with a rod down the middle of the cabinet. This meant that the speakers had a foam ring on both the inside and the outside of the speaker cone. When I opened the cabinet to see why I had a buzzing noise, I found that there was almost no foam still adhering to the cone. It had all rotted and flaked off. Essentially I had a voice coil moving freely in and out and causing no motion of the cone at all. What made this worse, of course, was that the drivers were totally enclosed and I couldn't even see that it was deteriorating. Other rubber on other drivers was sticky/gooey to the touch.
I looked into repairing them, and ended up junking them.