Rear speaker replacement little sound!?

I bought a 2004 RX-8 GT (fully loaded, only missing the silver rotors front and rear) so that means that it has the Bose stereo system. I didn't like the way the speakers sound so I decided to replace them. I bought a Rockford-Fosgate 6.75" comp system for the front and a pair of 4-way full range 6x9's for the rear. I put the 6x9's in the rear and they seem under powered. There's not much bass and they are more quiet than the original speakers in the front. Is there anything I could have missed by accident (it was a simple plug and play) or is the factory Bose amp just not powerful enough to push these speakers? Do I need to do a crossover or get an equalizer or anything like that? Thanks for your time guys/girls!!