You would be amazed how much difference even a small 8" subwoofer would make. I would suggest getting a small 5 channel amplifier, running extra power to your 4 stock speakers (which will make them sound better with less distortion) and adding a small 8" or 10" subwoofer in a seal box in the truck or hatchback. Total cost: under $500, sometimes including installation. Believe it or not, even the cheap manufacturers such as Boss audio and Pyle now make decent amps since most ALL car audio is made in China. I used to be a Boss audio dealer and had less defects in it than any other brand we carried (Pioneer, kicker, kenwood, sony, rockford, etc).