preout problems

Hello everyone ive been out of car audio awhile and need help. Vehicle 2003 Silverado Crew cab 4 door. I was given as a gift a new kenwood 210u that has only one set of pre outs it has the ability to run 85 hz/120hz/160 hz or through also has rev phase (180) or norm (0) heres my dilemma i...

Im having the hardest time trying to have both sets or rca preouts from the back of the head unit to be on full with no subwoffer.. i have one 4 channel amp controling 4 spkrs but only the front work and im getting a low "(subwoffer buzz)" sound comming from the rear spkrs.. anyone please tell...