Great thread!
If any of you read my introduction in the newbie forum you know i used to own car audio stores.
We were known as an spl store for the budget minded. When you only have 500 bucks to spend and you want a system that impresses, you really have to be careful what you pick.
We chose to do away with the 2 or more subs and design a single sub spl system. First we chose an amp. The Ample A-226x. Rated @ 130 rms x2 and 520 max output.
Then we had to find a sub and design a custom box to be as efficient as possible to make a lot of noise on a little power. The catch was ... it HAD to hit every note, not just be tuned to certain frequencies for spl purposes. The system must be able to go from Bach to Beastie to Bass-master.
What we ended up doing was finding a highly efficient aluminum driver by GS Designs. GS D class 12 incher.
Then we sat down with our calculators and three 4 x 8 sheets of MDF and 8 tubes of liquid nails... determined to build the box as many times as it took to get it perfect.
What we ended up with was a Stage 3 Tuned Port enclosure. The port alone houses 1.8 cu. ft. of airspace and runs 28 inches deep inside the box. It took 12 hours from the time the box was built, 'till the time it was finished tuning. We kept shortening the port, changing the width, changing the box size, untill we found the sweet spot.
What we ended up with was a $500.00 system that had a 260w rms amp, an alumninum 12, and a custom box that nailed 145.4 on it's maiden voyage at a local Circut City show with IASCA. (In a '92 ford explorer XLT with leather interior).
Keep in mind that the box held steady within 3 db from the bottom of the spectrum to the top on the rta. You could listen to anything from Phil Collins to Juvy and it hit everything full and clean. That's the difference, all SPL boxes are tuned for one note and sound terrible on some music. The 28 inch port allowed us to detune the rear soundwave 180 degree's out of phase from the front wave..... NO CANCELLATION! (notice below which way the port is facing).
I still have the original Stage 3 Tuned Port box in my X today. I just ran outside and snatched a pic, here ya go:
http://www.boomspeed.com/puppies/gssub.JPG
edit to add: We did not build SPL boxes, just loud one's.
because of the rules in my class, I can only have one additonal battery (no larger than the one under the hood) and the engine must be off!!! 
