box ? for the experts

I'm having trouble with a box I built - to keep from having to type it again heres an e-mail I wrote the manufacturer for advice:dB Drive technical service - I built an enclosure for my db Drive PL12D2 (12" 1500RMS, dual 2 ohm voice coils) subwoofer last weekend, and I adhered absolutely to the specs stated in your website. According to the tech specs, the ideal ported enclosure for the subwoofer is 2 cu ft, tuned at 35 Hz, with a 4" diameter port 13.12" long. I built the box, 28.5 inches wide, 11.5 inches tall, and 16.5 inches deep exterior dimensions, with 3/4" MDF. The sub is mounted facing upward toward the top of the trunk, under the rear deck of my BMW 635CSi. The port is facing the rear of the vehicle, and instead of a round 4" port I used a 10.05x1.25 rectangular port, which has the identical cross sectional area of a 4" round port. The port is on the bottom of the box and is also made of 3/4" MDF. I emailed you on November 27th inquiring the displacement of the woofer so I could account for it in box design, but you never replied so I found some generic information for a 12" woofer that claimed a good displacement assumption was .13 cu ft. The total box volume is 2.29 cu ft, but once the assumed volume of the sub and the volume of the port is included, it is almost exactly 2 cu ft. I have double and triple checked my calculations.The voice coils are wired in parallel so that the total impedance is 1 ohm, and it is connected to my Hifonics BX1000D mono 1-ohm stable amplifier.Now heres the problem. Pardon the childish language, but it sounds like poo. It sounds fine at low volumes but when driven to any volume much beyond a normal middle-aged womans listening volume it produces a sound that sounds alot like its being overdriven. The gain on the amp is below 1/3, the subsonic filter is set at 35Hz, but the sound is still terrible. I compared it to my brothers system, which consists of a $100 Jensen 300W amplifier and a $50 Pioneer 100W RMS 10" subwoofer purchased nearly 6 years ago, in a 1 cubic foot sealed box, and mine made his sound awesome. Its somewhat upsetting to me that after all the money spent and effort put into my system I'm being outdone with $150 and a sealed box.I thought something could be defective with the woofer, so I tried it in a different box. Another rectangular box with a slot port, it sounded wonderful. I then tried the above mentioned Pioneer 10" in the box I built, and it sounded great, infinitely better than the db Drive in the same box. So I've got two boxes and two woofers - The pioneer sounds great in both boxes but the db Drive sounds good in one and like dookie in the other.I thought then it could be the port. The inside end of the port was somewhat close to the rear wall of the box, and due to the large mounting depth of the sub perhaps the port was too close to the subwoofer. I redid some calculations and I found that the specs on the db Drive site do not match any formulas I could find anywhere. According to the most widely accepted calculations I found, a 4" x 13.12" port would tune the box at 32Hz, not 35Hz. I don't think this should have such a drastic affect, though, so its most likely inconsequential.So, I got a piece of MDF, some screws, and some silicone and blocked the port, got some PVC pipe and installed a 3" diameter 5" long port, which should tune the enclosure at 35Hz. I installed it facing up in the corner of the box, ensuring I had 3 inches between the end of the port and the subwoofer, and the enclosure wall. Much to my dismay, the sound is identical. So far, I've talked to some friends and came up with the following ideas for the problem:the sub is facing up, should be facing backthe sub doesn't have enough room in front of it (the rear deck is approximately 2-3" above the surround)port noise resulting from A) the short 1.25" height of the rectangular portB) the small 3" portNone of these problems would be too terribly difficult to correct, its just that I don't want to keep rebuilding my box until I get it right- I would much rather rebuild it only once and get it sounding decent.Do you have anything to suggest? I'm getting kindof desperate here, I don't understand why the sub works in another box but not this one, I can come up with literally no logical explanation for this.I apologize for the long e-mail, but I felt you would do best at analyzing the problem with a detailed description of the issue.thanksSo do any of y'all have any ideas on this? I, along with everyone I've talked to, is completely stumped.