How about theYou have 800W of power handling. All else being equal, to gain a 3db increase, generally considered to be the minimum audible, you'd need to go up to 1600W. Typically, though, higher power handling drivers tend to be less sensitive (less volume per watt) , so you'd actually need to go up to rather more than that. Then you also have to rebuild your cabinets to tune them for different drivers, at which point they'd be almost valueless. Wouldn't it be more practical to sell these cabs and find something that better matches your needs?
Eminence Kappa Pro 15LF-2 Bass Speaker (600 Watts, 15") at 8 ohms?