I just finished a custom subwoofer box and installed a 12" JBL driver powered by a 180 watt JBL amp. The manual said to make the subwoofer box 0.75 cubic feet so I designed the box flat in the shape of a wedge to conserve space. The volume of the box is exact, but the shape is that of a wedge. I lined the walls inside of the box with a thin layer of polyfill and used bondo all over the inside of the speaker terminal to make it air tight. When I fired up the sub for the first time today the driver seems to work very hard, and not much SPL cames out of it. The bass was clean and in phase, but when I turned up the volume the bass sounded raspy like it was bottoming out. I tried a different 200 watt amp and it sounded the same, even after adjusting gain and crossover settings. I am thinking of remaking the box in the shape of a square how most boxes are shaped and that my wedge type enclosure was flawed. I used high quality 3/4" MDF glued, nailed, and air tight caulked. This is my 3rd box and the bass is much weaker than my dual 10" and single 10" (boxy shaped). The JBL driver is a 10" 8ohm GTO rated up to 275 RMS. So My assumptions are telling me the box is crucial and if the box is shaped irregularly then it may not produce the right bass. BTW, from a side view of the enclosure, there are no 90 degree angles... (were talking a slanted trapezoid shaped box).