( not quite sure what u mean, dlayen... RE: "there must be a line level going in to the amp ..etc etc. ..")
the head unit does not have an amp in it so the headunit output's at line level (0v-4v normally) to the amplifier to amplify for output to the speakers.
so there must be a line level output from the headunit that you could tap in to to drive another power amplifier for extra subs as requested.