Last summer I put a Sony X-Plod Head Unit (52wx4) and some 6 inch speakers in my truck. The speakers sound fine, but they don't have as much bass as I would like. It would be nice if I could swap out the speakers for subs, but I really don't have the money or the space for an amp. Do I really need one for just 6 inch subs? Recommendations on amps/subs would be appreciated, too.