i've been messing with my rear speaker for the past week or so. i have them hooked up to the front output wires from the hu. they seem to pull the stage to far back and the music just doesn't sound right with them on. i've tried turning the gain up for the front speakers and that helped pull the stage up front more, but i still don't like it. i even tried putting a couple layers of ensolite in the inside of the door speaker grill so i can get less output from the rear speakers, but it still sounds a little off.
i do like the fullness and more mid-bass that the rear speakers give though. the sub blends in perfectly, like there isn't even a sub back there.
after about a week, i'm back to just running my front stage only. i like the clarity and imaging better.