I used the pre outs on my new head and rewired the OEM amp and I now get left/right and front/back control of the speakers. I did it because I could as I am pretty deaf so it is pretty much wasted on me, the rear speakers can feel like a punch in the kidneys, but they would not last long at that setting I am sure.
It was awkward to do cutting and splicing wires under the dash at my age but it would be much easier when replacing an amp. Selecting the wires to cut was a pain at first as the colour of the wires and the colour on the wiring diagram could not be taken as gospel, no mistakes though.