This came up in a subwoofer forum once.
I haven't read up on it in ages, but the best idea presented at the time was Vaseline or KY Jelly on the foam surrounds.
Doubt that helps in this case though.
I know someone from the subwoofer company posted that it should last at least 10 years, FWIW. (note; they did not make the recommendation of ointment)