Alpine 9887 Pre-Out Voltage Problem

hey everyone
ok heres the problem

i finally connected everything together, and there is no out put from the sub, until gain and volume are at max setting.

i tested the pre out voltage from the head unit, and it goes as follows

front 3.364v

rear 3.410v

sub 0.599v

why is the sub out put so low, is there something very wrong with the head unit or some thing else.

thanks

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