It wasn't the PD speaker that had two cones, it was the B and C speaker and it wasn't in the MX1200 either. If you have blown them up through sheer power, then a stiffer cone won't help. The 1550's were standard. The speaker I would use now is the B and C 15PL100. I can also supply you with these, should you wish to go down that road. Sorry to hear such bad news on the speaker front.
so for the mx1200 a standard pd1550 is fine? would you say that the b&c is a a upgrade and how much are they as it is only 65 for a 1550 re-cone kit. we was only running say 1100-1300w in to a cabinet off a poewersoft k8, but we had ran them down to 42hz trying to not use subs under them smiley21 . we have learnt are lesson now for sure.