Sir dont power the rear speakers, that way ur sound stage might shift to the rear, which you dont want.
If i were you, 1st thing i wud do is add a sub in a sealed enclosure, power it with an amp. if properly installed the sub will merge so well with ur setup that you wont be able to pin point that from where the sub bass is coming from. Select a SQ sub such as JLW3v2, costs around 5k. adding a sub will make ur setup sound richer n fuller, if properly installed n integrated.
after that i will put some good 4 ohm speakers in the rear factory location and run em through the head unit.
All done