I think you should should replace the rear subs with regular speakers. The OEM amp is 4 channel. Beware that the rear cannels of the OEM amp are set up to run dual voice coils. There is no aftermarket 6.5 subs that have dual voice coil. I installed some kicker's and they are not as loud as the OEM speakers becuase of them not being dual voice coil. I also had to do modificaiton to the plastic basket to get them to fit.

https://www.crossfireforum.org/forum...rear-subs.html

https://www.crossfireforum.org/forum...amplifier.html