I personally like using cheap subs to compete because it shows that the box is more important (to a point) than the subs, and its fun to crush people that spent 3 times what you did //content.invisioncic.com/y282845/emoticons/laugh.gif.48439b2acf2cfca21620f01e7f77d1e4.gif
There is some good advice offered among the flaming here...you should upgrade your wiring to 1/0 and do the big 3.
If 150 is actually your goal then you'll need to upgrade your amp, subs and optimize your box.
Similarly, I run cheap subs ($100 each) and a cheap amp ($150). It's all about doing your research, planning ahead, and buying smartly.
Hitting 150 isn't possible, accept that...but 145 should be a goal to shoot for. Honestly, I would be happy in the 140-143 dB, TL legal, range.
Bear with me, and I'll do what I can to help a learning SPL nut.
In a Grand Prix (with all trunk cars), you have a few options, and I'd bet you can get up near 143-145, with that equipment and spending some time tuning. Here are some box choices:
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Standard trunk will lose a couple dB's instantly. If you're trying to squeeze every bit out of your setup, don't use this.
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Rear-deck/middle-seat blow-through is good for staying in stock classes for competition. It also keeps things secure from thieves. This would be my choice. Either cut a hole, or use the existing speaker holes.
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Sealed trunk is as loud as you'll get, since you're basically walling the trunk off. It takes some time, but allows you to keep your back seats and compete in either stock or unlimited classes.
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Wall is impractical for most people, but it does get crazy loud and takes alot of time.
1) Choose your style, and setup the basic structure.
2) Build a test box, with ~3 cf sealed box.
3) Using a meter, run 2 tests. One using a sine sweep with the car sealed up, and one with the windows, trunk, and doors open.
4) Find your cabin gain peak. This is the where the biggest difference between the 2 tests happens.
5) Build a ported box, with 4 cf IV, and a large port (200 sq in) tuned to ~3 Hz less than this frequency. Try to keep port and speaker facing forward. Double-baffle the front, and use lots of wood glue and screws. Apply a layer of duct tape or fiberglass to the inside seams.
6) Ensure your wiring is proper and gains are set properly (to a light clip). 1/0 wire is needed for your amp, along with a clean ground, and a quality battery.
This should be enough to get your setup to 138+, TL legal. Everything from there on is small tricks and tuning that comes from lots of testing.