that unit has a separate sub control. have you turned it up? Also you cant just jump the rca,s for another amp because the ones going to your mono amp are most likely just sub outputs and no high frequency's. you have to run another set of rca,s from the head unit.
I know, If I got the 4 ch amp I think I could just set the RCA's as rear on the deck (I only got one RCA outs), then use the output from the 4 Ch to the mono block and use the crossover at about 80Hz or so.
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