5x7 vs. 6x8
5x7 is a standard speaker size. 6x8 is something that Ford started. Back in the 70's Fords had 6x9 speakers in the rear deck. I think Ford started the whole 6x8 thing, to reduce the width of the rear package tray. Also putting (good) 6x9's in doors requires a DEEP door. 6x8's require less depth, which means that thinner doors can be made, which saves weight.
The JBL sub is a mono 6" driver, so you can upgrade the JBL driver with something more robust. I bought a JL (might as well go with the best) 6" subwoofer ($75 on sale) & installed it in the JBL sub box. Since the JL driver is deeper than the JBL, I had to modify the box a little. The sub box has legs that stick up inside the box. These legs support the magnet of the JBL driver. If my memory is correct, the JL driver is 3/8" deeper than the JBL. So I had to cut 3/8" off the legs. The box & legs are fiberglass, so they cut easily with a hacksaw blade.
Since I was already inside the sub box, I added some extra features. I installed a set of input terminals on the outside of the box, & a DPDT switch. This gives me the option of using a different sub amp, & switching between the JBL & external amp. There is enough room inside the box, that you could install a Low Pass Filter (LPF) of your choice.
Also there is a large foam plug in the output port of the sub box. You can experiment with using it, or removing it. Personally, I found that I liked the sound best w/o the foam plug in place.
With the heavy duty JL driver, I also found that I had to glue the rear package tray carpet, to the rear package tray. Without glueing it, the carpet flopped up & down with the movement of the sub.
68COUGAR