ppi 6 ch ax606.2 problem issue?

installing a ppi ax606.2 amplifier. using an alpine cda-117 headunit with front rear and sub outputs. the issue i'm having is the subwoofer channels (5,6) or getting signal from either the front or rear input (1,2,3,4). there is a combine button on the amplifier, but does not change the issue of the subwoofers getting signal from another input. does anyone have any insight on this or perhaps have seen it before?nice...

That amp has only 4 inputs.  It was built before 3 sets of RCA outputs were commonplace.  If you read carefully, the other set of RCA jacks on the amp say Crossover Output.  I think the combine switch combines the front and rear inputs incase you have a radio that has only one set of RCA jacks.