The beolab 6000 speaker woofers do not use foam rings. From new, the speaker surrounds are a very flexible, thin rubber which do not suffer from foam rot.
However, it's not all good news. The rubber surrounds do harden and split with age and the sound quality deteriorates markedly.
Foam replacement surrounds are marketed by Good Hi Fi among others. I have heard Beolab 6000's with foam repacement surrounds and quite frankly, they were awful! Bass was well down on quantity and the speakers sounded "dead".
I cursed the guy that sold them to me and replaced all 4 with B&O service replacements. It cost me £150 which I had not expected to have to spend but the sound quality was back to "as new".
Since that time, I have had to replace many, many woofers in old Beolab 6000's and unless they come as part of a package deal and are very cheap, I no longer buy 6000's for re-sale.
There will be those that will disagree with me but I would say beware 6000's that are more than 10 years old. They could cost you more than you think!!