I used to have a down fire setup in mine and had the box built by a local shop. I've since designed and built my own box that faces the front because the down firing box was too large, and the amp rubbed a small hole in my Wet Okole seat covers (not the ones in the pic)
As far as sound quality is concerned, I didn't notice much difference between the 2 setups. One thing I did notice was that my subs didn't seem nearly as loud in the down firing box, but that is probably more attributed to the fact that the boxes don't use the same amounts of air volume.
My old setup:
New setup. The wires and carpet have since been cleaned up. This was when it was installed: