UK - replacement speaker wadding question

[surv1v0r]

I have a pair of B&W DM2 loudspeakers which have transmission line ports. In about the final two thirds of these ports, they use a fairly open, stiffish wadding that can best be likened to a loofah or a scouring pad. Over the years this has become rigid and it now sheds fine powder. Aside from the resulting mess, I suspect that it has ceased working effectively. I guess that it is about 50mm thick by 320mm wide. I haven't been able to identify this material or to determine whether it should be stiff. Nobody at Bowers & Wilkins, Wilmslow Audio, Falcon Acoustics or elsewhere seems to know what it is. I would very much like to source a replacement - can anyone advise what it should be like and where I might be able to buy replacement material?