how about wire both of the infinity's in 2 ohm and turn the gain all the way back (lowest), set cross over around 50hz, turn off and bass boost on amp, lower the bass on the headunit. this should at least solve most of your problem. if you have a 4V preamp output on your headunit, forget what i said
u can add a 15" on it, just make sure you have DVC on it too so it won't confuse you that much.