Alienturtle,
I'm sorry I just notice the thread, I have few questions: Did you put a layer of 3/4" fiberglass insulation matt glued inside the box, at least in one of every opposite walls? If you want to avoid some kind of reverberation at certain frequencies (in the mids and highs) you will need that.
I'm also worry about the possibility of the fiberglas wall vibrating and making some noise against the floor. I will put something there in between to avoid that.
Also, do you plan to have the seat removed most of the time so you can hear your music with good quality? With the woofers is ok to place them with the seat on top, even inside the box, if you turn the box into a bass reflex type enclosure but the 6 x 9 would give you a better performance putting them in the front wall, toward the ends like you have it. With a protection grille in front to avoid somebody hitting them with the back of their feet they will be OK, and you can have the seat on all the time. I know is not a good thing to go back to fix things in a project when you think is almost done but believe me, you will notice a big difference.
If the 6 x 9 are going to handle just the mids and highs (which is what they are good for) you need to isolate them from the rest of the box where the woofers are not only sealing that section real good but also with a crossover (one for each channel). If you don't have the crossovers or cannot find them I can help you sending some instructions how to put together your own with few components from any Radio Shack store.
With just these few changes the system is going to perform great. You have done a great job with the box and the fiberglass to use all the available space under the seat.
Burt.
