Why does he have 6 12's with that little of power. Does he not listen to you or any body for that matter. With that much money and room he cood have a nice pare of 12's! What kind of car does he hav to fit 6 12's? I didn't say any thing about volfenhag subs!
ok an idea on SPL.. 130dB is the max IASCA events give points for since they base system judging on SQ, install grade, and creativity more than raw SPL. 120dB is IASCA peak points in the lower classes like amateur and novice if I recall. 115dB isn't bone crushing, but it's loud for sure. If you're using a single 15" with a JBL bp600.1, make sure the sub is 2 ohms, or dual 4 ohm coils, and wired in parallel. use a ported box, about 3 cu. ft. and use a 6" port, tuned to 28Hz. that'll make that sub hit pretty hard. angle the face, or back of the box so they aren't parallel to avoid cancellation. be sure all 4 inputs on the JBL amp have signal by using an RCA y-splitter pair of cables to split the two RCA lines to four. that helps a lot with that amp. make sure amp gain is set right..
that's about all the advice I can offer there.
If you can get into the 130s with that sub it'll be pretty good. If you want better, think about going to a better sub, too.. one with a greater Xmax perhaps to move more air.