You don't need an amp.
If they are playing and their just seems to be a lack of bass, you most likely have the polarity reversed ( + and - backwards).
Fade and balance to zero.
If when fading to the back, the bass increase, the rear speakers are out of polarity...both of them.
If you fade all the way to the back and then as you balance side to side, the bass increases, the one of the speakers is hooked up backwards.