What size is the subwoofer?

In my experience, a 10" or smaller subwoofer running in a free-air environment (i.e., not in a sealed box) is pretty happy running full range frequencies. In that case, you could just run it off the rear speaker outputs, assuming that they are not crossed over with a low-end cutoff.

caveat: My experience is entirely with automobile stereo systems, maybe home audio is different.

tanstaafl.