Have you considered a powered subwoofer? Its a subwoofer with an amp all in one. I got this kenwood one for my truck and i like it a lot, and i believe its a good deal. Its smaller than what youre looking for, however it packs quite a punch for its size and i find it does a good job filling in the lows for the type of music i listen to, which is rock/punk. Also, crutchfield.com is a great place to look, the prices are high but they have great customer service that is very knowledgeable.

http://www.amazon.com/gp/aw/d/B00BO8O9XQ/ref=mp_s_a_1_1?qid=1416187659&sr=8-1&pi=AC_SX200_QL40