I was thinking. The speakers in the rear don't really do much for sound stage. Plus, there is not a lot of room inside the cab for woofers. Is it possible to install high quality components upfront, then 6.5" (mid woofers) in the back, and only have the amp under the front seat, retaining cargo capacity in the rear. The amp would filter only base frequencies to the rear. Question is, can you get 6.5 inch speakers that come close to sounding really good for bass only? How do you think this would sound?
2001 F150 XLT Xcab.

 

I totally agree with you concerning the uselessness of rear speakers in the supercabs... I've tried three types back there with 60-75 watts and finally found the best balance I have tried. I have a high end set of components up front and only silk tweets for rear fill.
6.5 woofs won't give you earthshaking bass but, mounted firmly with good wattage will give you some low end fill. Personally, if you don't want to take up much room, a sealed sub, 8 or 10" with moderate wattage would do you much more good. Try a 8 or 10" JL W0 with about 100-200 watts.
I say all this with the thought in mind of a sound quality install. Crisp, clear and with impact..... not just loud noise.
Good luck and you seem to be on the right track....

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1997 XLT Supercab, 2wd flareside.
Very bright RED.
Exterior mods:
4x4 bumper and fogs.
Stull Billet, bladerunner, Hyper lites, pro-beam covers.
Bugshield, window tints, vent visors.
Pirelli S/T Scorpions, gas shocks, 2 inch drop, slash duals.

Interior mods: Audio System
Clarion 9375, 9300 EQ, 12 disc changer.
Amps: Rockford Fosgate Power 500, Soundstream Picasso.
Sub: Single 12" JL W3D2 under rear seat that was raised 4 inches.
Sats: Front; Diamond Audio Hex s500.
Rear Fill:MA Audio silk tweets.