Well, I'm not an expert on SPL, always more of an SQ/SQL person. I do know physics pretty well. If you want outrageous volume, you use the subs you have, feed them 6-10000 watts in a 5.5 cu ft box, tuned to around 34-37 hz, even louder at around 63 hz. You don't get both without paying, A LOT! Everything has it;s forte and most things are mutually exclusive unless you pay a boatload of money to accomplish both. you may want to consider going to a couple of 15's too. 140db is pretty lofty for subs under a grand each, it's the dollars to watts ratio, not in your favor. Tuning is consideration also, SPL subs are often tuned inthe 60hz region too, not a great sound but it gets loud.
There are many in here that will suggest alternatives, but here are a couple:
The CT Sounds MESO-12 is a 12-inch car subwoofer capable of reaching 3000-watt max power. Available in dual 2-ohm and dual 4-ohm coil configurations.
www.ctsounds.com
ina 1.5 cube box tuned to 37hz
If you're into gut wrenching bass during the week and competitions on the weekend then the LAZER PHASE THREE Subwoofer Series is just right for you. White powder coated Strong cast aluminum frame, CNC large vent motor structure, Free Flow Cooling System, double stacked Y-35 ferrite 100 oz...
lazercaraudio.com
THis is also a great "loud'sh" sub:
The Hawk 15" Subwoofer is the ultimate answer for car audio enthusiasts & SPL competitors! It's not your average subwoofer; it’s a revolutionary powerhouse.
www.americanbassusa.com