Rear speakers are barley even noticeable when you're in the front seat. I wouldn't put anything in the rear that was a lower quality speaker than what I had up front, that would be pointless. I tried it once and the sound from the rears bled through a little and just ended up detracting from the sound as a whole. Personally, I'd only do it if the rear speakers were matched to the front ones, even then it isn't necessary. I had Diamond hex components in the front and rear doors of my last car and when I turned the rears off with the fader, it was BARLEY even noticeable up front. Your sound stage and presence will benefit far more from adding some 5 1/4 components in the kick panels than adding rear fill. That's what I'd use the extra power for.