I did "kinda" the same thing you are planning.
I did Hertz HSK165XL up front. And they are running off of a RE Audio 4 channel amp, bridged to a 2 channel. Factory amp is still powering the factory speakers, but its tuned so about 80% of the sound is the front doors.
I have tossed around the idea of doing some Hertz 6x9s in the rear deck on a small amp, but then disconnect the rear doors, and send that signal to the rear deck, that way they are getting a HP signal instead of the factory LP signal. The stock 6x9 "subs" are disconnected since I have real subs. And I have people in the back seat alot, and doing a set of 6x9s on Full range would add a lot more sound than just redoing the rear door speakers.
I got a wild hair and wanted to try out the new Retro Style Alpine subs called Bass Line, they just came out with. They are the replacements for the Alpine Type E subs that Ive always really like the sound of. So right now Im just running two 10s in a sealed box off an old Pioneer sub amp. Its 1000RMS at 1ohm, so I keep it around. lol
If I decided to run full range to the rear 6x9s and replace them with a set of Hertz speakers, I might just sell my RE Audio amp, and buy a lot bigger 4 channel, that way all 4 can run off the same amp. But the RE Audio amp I have right now, as a 4 channel isnt enough power for the Hertz, that why Im running it bridged as a 2 channel.
As far as boxes go, I am currently running just your standard MDF box, but ive been looking into doing a fiberglass box on each side of the truck(single 10 on each wall), because right now, I usually take my sub box out in winter, because I need the pass though for snowboards and ski's. So doing a sub in each wall, would allow me to leave them in all year around.
