As you are aware, the head unit has 2 sets of outputs, front and rear/sub. Unfortunately the rear/sub output is an either or. I would use it for the sub as you mentioned. To drive your door speakers, I believe you have 2 options. Use 2 "Y" adaptors and split the front output and use your amps level adjustment to set fade. Or my choice would be to drive the rear speakers off the head unit and keep the ability to adjust fade from the head unit. And if your 4 channel amp has the ability to bridge you can send more power to the fronts.
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