RE Mine looked like that a few years ago I just use a damp terry cloth rag and very carefully wipe the rubber. They look like this now. I did Google EPDM Cleaner and found the following - My advice is only use a damp cloth with water don't bother with any cleaner - as if there are issues there is no way back!

+1, same as christo

Did the same thing, couple of years back, on my 1977 Polk Audio Monitor 5 rubber surrounds and they came back pretty good. These are also essentially unobtainable. The before and after cleaning pics were posted in my BGW Amp thread. I still own and use them (about 47 years now) and i'd do it again if i have to. Can't kiss them goodbye...

Richard