B&W 600i speaker sounds weird..

One of my lovely B&W speakers suddenly started to sound weird. The bass would come on and off with certain music I played through them. At first I thought it was my old Luxmann L-2 amp that caused the problem but the other day when I took of the front of the above mentioned speaker I noticed that the outer rubber membrane had turned concave instead of convex. How could that have happened? Is there a way to fit it back into shape, or,- has it gone to the eternal loudspeaker heaven?

The spec. for my old Luxmann amp says it plays 33 watts per channel. Are these 8 ohm 100w speakers the right size for an amp like this?

I look forward to hear your advice.

Thanks

Peter