i second mello's post. ditch the rears no matter what. if the fronts are good enough and you don't need big bass, you can pass on the subwoofer too.
my old s13 had infinity components up front (i put the tweeters on the dash though) and 4x6s in the back. once, while messing around i turned the fade all the way forward and i couldn't even tell a difference. the front speakers so thoughly overpowered the rears they are a complete waste of time.
if you really want a sub, try this. since it looks like you have a coupe, you could put a pair of 8" subs on the rear deck (JL and Memphis make a few). nice addition to the low end without a lot of lost space or clutter in the trunk.
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