i found a really good cheap method for this. i bought boka brand computer speakers with a subwoofer, its the 850watt version, that can accept rca's & speaker wire in the back. the subwoofer is the amp and has volume and bass controls on the front, it can power 4 or 8 ohm speakers and sounds very good with car stereo speakers or the ones that were in the jamma cab. the speaker package runs about 40$ it comes with 2 spekaers that i just use some where else as well. i find them on ebay all the time just make sure you get the 850 watts as i think its the only one with speaker wire and rca's the other lower wattage models are minijacks in and speaker out. i have bought 2 and they run both of my cab perfectly and add a great amount pof bass for music and movies too.
gnat