[Arran]

You could try freeing up the frozen voice coil, but that would involve removing the spider, cone, and voice coil, to do it properly if dirt or rust has caused it to seize. Sometimes they seize up due to cone warpage, which can be fixed by inserting some thin but stiff material as shims between the pole piece and the voice coil, and then by wetting the cone and spider to re-align it. If you want to remove the whole cone just soak the area around the outside of the cone with lacquer thinner, the same goes with getting an externally mounted spider out with the cone and voice coil, don't forget to unsolder the voice coil leads first.
Regards
Arrran