Sub Pre Outs

[fivehundred]

I purchased a cunch PowerZone 5000.5 5 channel amplifier 4 x 125 + 1 x 300 @ 4-Ohms 4 x 200 + 1 x 500 @ 2-Ohms Maxx I am running 6x8 pioneer 240 watt speakers in the front and rear of the car along with a 12' single subwoofer, my problem is that when i hook up the sub rca's to the amp it gives me a constant noise through the front and rear speakers, is there any kind of setting in the head unit that turns on the rear sub rca pre outs or are they constant 4.0 volts, the front and rear pre outs work fine i dont get any kind of noise at all with just them hooked up, any help would be appreciated