The amp receives the output signal from the head unit - whatever volume is going into it is coming out of it. If you find the bass is overpowering the system dial the gain back on the amp controls. I'd think if you add 6x9"s up back you will find the bass will become overpowering. A 'good' basic install is a pair of 6" splits up front (6" driver with a separate mounted tweeter - this is where the cross over unit is installed. It separates the midrange speaker from the tweeter speaker so they speakers are getting the right sound frequencies pushed through them), 4" to 6" dual cone speakers up back (this is to give the system some rear fill) and the subwoofer in the rear of the car (normally facing towards the farthest rear corner away from the drivers position) You can see the inputs on the amp (power, earth, rem) the 'rem' is the remote trigger input that runs from the head unit (this tells the amp to turn on - it is a low power input so don't try to hard wire a 12v feed into it). Where you see the 'bridge' marked on the outputs, this is the recommended method to hook up the subwoofer (positive from one channel, negative from the other). The amp that you're using is designed only to receive 2 channels from the head unit and is only going to be used to drive the subs so you can't mess that up.