There a few other techniques we can use to increase SPL like rebuilding a better box or stuffing it with poly or glassing the inside etc… The best way to get more SPL is to get more woofer area. Then do sound damping and finally get a big nasty amp. If you’re like me you’ll never stop making your system better. Just wait until you want to get the best sound quality possible. It’s fun but requires a lot of time and good gear.