My suggestion is to leave your existing speakers connected to the head unit as they are, and leave your amp just for your subs. Connect the RCA leads to the rear output on your headunit, and at the other end have a 2 x RCA plug to 1 x RCA socket lead (click here
), one each for left and right channels, and hook them up into the left and right channels of your amp.
Use the 4 channel amp as a 2 channel amp for your subs by bridging the left channel for the left sub, and bridging the right channel for the right sub. Adjust the settings to suit and away you go.
Hope this made sense...