you can do what you want but if i was in the position of spending possibly $200 id just invest it in the fronts. if you keep the rear just run them off of head unit power.
what i would be getting is
speakers
Image Dynamics CTX-65
amp
Precision Power PPI S420.2
what i would be getting is
speakers
Image Dynamics CTX-65
amp
Precision Power PPI S420.2
