Subwoofer Rattle

I have a B&W ASW850 subwoofer that is rattling in the 20-40Hz range (tested with a signal generator). Although I have never consciously over driven the unit, I assume this just means the voice coil is shot and I need to replace the speaker? How easy is this to do?

Thanks for your help.

Hmmm...if the whole thing needs to be replaced...sometimes they do that...the voice coil, magnet, etc...that probably is a good portion of the sub. I think it is pretty straight forward...just might be expensive. i am NO i admit i could well be wrong, but i have replaced entire cones, a tiny inexpensive one can be 20-50 expensive one can run a few hundred.
Sounds to me as if you will have to replace the drive unit. B&W has them in stock here:
What do you mean by rattling? Have you tried to push on the cone to see if the voice coil is actually touching the magnet? If you have never driven it hard, it is possible that the problem is in the built-in amp and not the voice coil.