Some info on the units can be found
here.
The surrounds have turned into something resembling brittle plastic; some parts have fallen off and the rest breaks easily.
I got some replacement surrounds from
repairyourspeakers.com.
The instructions can be found
here.
Repairing the woofers was not as hard as I had feared, the instructions were clear and simple.
The old surrounds broke into pieces, which were simple to remove. I didn't risk trying to get the final bit off of the cones; the risk of damage was too great. So I just glued the new surround on top of that.
Getting the cardboard layer off was trickier, it was really glued on well. Some sticker remover and careful scraping took care of that.