i am in a league in new york called team northeast spl and i have an 07 impala and currently have 2 12" powerbass 3xl subs hooked up to a powerbass 4000xa amplifier and my numbers currently have been 146.9 and want to do alot better then that i also have a kinetic 2400hc in the trunk am getting an 225 amp alt soon and will do the big 3 when i get back from iraq i hope this is not to big of a question just want to be as loud as i possibley can thanks for any advise you may have.

The only way to increase SPL is by doubling the power to your woofers. increasing power by small increments won't really be noticeable when measuring spl.

doubling power would probly be my last thing i personaly would do. I would try a different box design, or move the box around. I recently built a 15" setup for a bmw 325i. The first run was a 143.2db. Then the box was rebuilt to a spec beyond what the company told us. Now the setup does a 147.2db at the headrest. And this is all on 1 15" Audioque hd3 with a audioque 2200d (clamped 1500watts at the sub)

Yup like everyone is say only adding two more subs will add more spl, I have my self competed for many years. I started with hitting 148db then by adding acouple things boosted my spl to 150db! Be sure you have enough of dynamat (Hush mat is better) For a car that new I would double all your ground straps from the engine to the chassis with 0 gauge because that car is only made to run on its OE components. So beefign up your ground straps and running your power wire directly from the alt. Oh yea I only recommend getting alts from Ohio generator, there the best in the spl alt business, period! Been in it for years....Box is really important too, make sure you have no rough edges in the box, what I always do is put fiberglass in all of the seems to give it smooth flowing corners, dont want any port turbulance. I have also seen a increase of db by adding a sound processor(boosting your signal voltage) be careful if you dont know to install it you will smoke your amps, seen people do it on a regular basis. Deff. do the wiring first you will hear a difference, promise!