The amp should be connected to the rear output of the head unit to achieve this. Keep in mind that this could also be controlling the volume of the rear speakers.
My amp was wired to the subwoofer and the rear sound knob on my stereo controlled the volume of the sub. I don't understand how to wire it up again. I wired in the sub in a simple bridged mono output but this doesn't accomplish the old setup like I had. I have a Jensen sub and a Pioneer GM-X572 2-CH amp. If anyone could help me, it would be greatly appreciated.
It doesn't control the volume, it controls the gain which is completely different. It changes the frequency in hertz, putting the sound in a different audible band. Well, more or less, I'm not an audio nut.
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Erm, not really. Gain, sure. Frequency, no. But the rear volume know I am guessing is being referred to here is probably the fade (front/back) knob, and hooking the sub amp to the rear speaker outputs would control the level of the sub with the fade knob.