Ditto!!, AMEN, X2...If you're getting the proper imaging, balance, etc, from your front speakers, THEY will provide the rear fill. The rear fill from a live performance comes from the stage. Also running rears in stereo will hurt the front sound stage and imaging. Do a search on rear fill, you will find many others who say the same thing.
You would be better off getting better front comps for the front, better speaker placement, or more damping/ better install, and also sound deadner! depending on what you need.
Not to take anything away from what shpatb said, but if you own or have used a good set of Head phones, you should know why ONE pair will work. The stereo recording is only two tracks. When you listen to a a great recording with headphones you will notice that the soundfield is not from the rear, but from the front. Even though both speakers are directly from the side. This is why one pair in the front is ideal. If you go adding speakers, indescriminately to the soundfield, without DSP, you will smear or blurr the soundstage image. You MUST have surround sound processing, or the extra speakers are a waste of time...period!yeah two sets of the z's is intense, and way over kill. You should of just gotten a boston amp the 24 or 28 perferably, I have the z6's in the front and that already kills my ears, anything more then that would just be stupid, and ruin the sound stage of the car. You're better off geting the w7 or 555 and calling it a day. The SPZs wont let you down, but two of them is too much. like the other guy said, spend the money on something like sound dentening or better amps... gl.
Yah thats a good idea too!Return the 4 channel, and get yourself a strong 2 channel, or 2 smaller 2 channel amps.. ex. 2 sundown sax100.2's. Run the 2 channel amps in dual mono to each set, those z's will love 400 watts!!