Even though the 928 S4 has a 10 speaker system (drivers were even updated 2 years ago), the overall system lacks any serious bass. I wanted to make a system that was easily removable, easy to adjust the settings, unassuming (didn't look like a subwoofer upon 1st glance from "midnight shoppers") and was reversible (should I ever sell the 928).

Yes, the box I made could have been modified to actually be a sub box (and add separate amp/s)...I chose to get an all-in-one amp-speaker...plus, I was trying to avoid speaker grills facing straight up.

These 10" units are from Boss. They don't play extremely low, but they hit hard.

The entire enclosure was made from one 48" x 24" laminated shelf from H.D.

Here is a link to a short/crude video:   928 Subwoofer Demo                                                                                    I didn't post this on YouTube because it was so basic...plus the subwoofers overpowered the small mic on the   iPhone a few times (distorted).  If you don't have good headphones or listening thru some nice desktop computer speakers (preferably with a subwoofer), the video will be worthless.


Next 4 images are the finished product:






Rear seat backs "up"....


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...and it begins!   1st step is tracing out a pattern. The Styrofoam piece holds the cardboard up at desired height (since back area tapers a bit).


Tracing...




Cutting...


Hole sawing...


Assembly...


You will note some small notches (1" x 1.5") on the back of the enclosure ABOVE...these slide into some brackets in the back of the 928 (BELOW) that were originally tie-downs. This worked out perfect to semi-lock the unit in place during driving.






...after painting


Small groove for round buttons in back that hold tool kit...


Gluing down carpet...




Test fitting one subwoofer...


Start the wiring...






Pulling wires from back to head unit...





Subwoofer volume controls are the 2 small units in the shelf area with the blue lights...near center of picture.



Unit easily tilts up for access and adjustments








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*Update*     I removed the screws holding down the fans (transmitted too much noise to the enclosure) and used high temp silicone instead....I used weather seal strips between the fan and wood shelf (to further decouple).