I put a pair of Cerwin-Vega something-Max series 4" speakers in the back. I've since blown them away being on the same two channel amp as the 6.5's in the front, but when I had them, it makes the sound much richer, less like wearing a set of headphones and more like being in a music hall with the sound encompassing and vibrating everything in the car.

Of course that might have been the pair of 12" subs I have.
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