Hey man the first thing I would do is clean off the old glue, use some sand paper to ruff-up the metal frame of the sub-woofer and then use a two step epoxy glue, you can find it at Wal-Mart or Traget for like $5 a pack. Their are a phew different types of epoxy you want the one that has a dispenser that attaches to the end of the tube. As you push the plunder into the tube the epoxy mixes as it comes out. You will be able to put a nice line of epoxy on the sub-woofer frame and then press the cone in-place.If you have some c-clams you could clamp the cone to the sub-woofer frame. If you do clamp the woofer cone to the frame I would place a piece of wood like a section of a wood stir stick under the C-clam on the woofer cone side so that when you clamp it down it distributes the pressure evenly.
When you are shopping for the epoxy if they have the Lock-Tight brand then they most likely have a quick set epoxy that sets-up in 1 minute and another that setts up in 5 minutes. The package says that the 5 minute epoxy can hold like 2600 pounds of force and the 1 minute epoxy can hold like 2100 pounds or something like that so I would get the 5 minute epoxy.
I have used the epoxy to fix many things and its worked very well, just be sure the surfaces are ruffed-up with sand paper, dry, free of dust and very important ones you press the two sides tougher don't move them very much, put it in the right place and clamp it and let it set-up for hours or over night for best results.
Hope that helps
