Or get one of the Helix DSP amps.
These are plug and play but change the EQ to flatten out the factory head units EQ and then boost it where it is really needed.
http://www.dynamicsounds.co.uk/helix...e-p-25215.html
However, you have the business head unit, which has a flat response compared to the professional, so you don't need anything too fancy to flatten out the frequency curve.
Also, just make sure the switch on the amp is set to Position 1, this will give the subs a little more umphhh.
Your individual system did have a lot of info going to the rears, but this was mainly because even that is not quite good enough to just have the fronts playing, so they used the rears to fill the cabin, a pretty good compromise.
Earthquakes are nearly drop in. You have to seal them in the oem cabinet. But trust me, I have been through this so many times on so many forums, every one resists and as soon as they have done it they all agree they should have done it far sooner.