Are you asking about replacing the drivers in the existing enclosures?
I personally doubt that would get you what you want (higher SPL or lower distortion at current SPL) at a reasonable return on investment. Better drivers exist, probably, but the value component (cost vs performance) is hard to justify.
You should figure out if replacing both subs with two new units is cost-effective, or if replacing both with just one new unit will do what you want. Subs have come a long, long way since 2001, or even 2011.
The resale value of your JBLs will be greater in stock form; modified gear generally sells for less than unmodified.
Then apply that money to your upgrade path, which might include just better, used subs.