Turns out the spider had come un-glued from part of the cone, the voice coil is undamaged. In the end I purchased a new subwoofer from B&W ($700) and everything works fine now. Allen Speaker Reconing in Houston are repairing the original speaker for me now and I will either keep it as a spare or sell it on.
Re-winding B&W 15" Subwoofer Speaker
I recently posted a question about a failed 15" subwoofer speaker (B&W ASW850 subwoofer). It has just cost me $700 to buy a replacement speaker from B&W, everything works fine now. It would appear the coil on the damaged speaker had become loose, causing a lot of rattling at certain frequencies. Does anyone know if there is a repair center that can rewind this speaker voice coil? The speaker part number is ZZ14102. If I can get it repaired, I will keep the old speaker as a spare, or maybe sell it on.