I personally would get rid of the sony xplode and get a pioneer headunit. You get the most options for your hard earned $. Not the greatest in sound like alpine but there are so many freakin' fine tuning abilities that you have with them it makes up for it. But, I would also run better speaker wire to the speakers. If the car is like mine all you need to do is take off the cd player trim part and try pulling it out. Most likly there are "tabs" on the metal sleave around the cd player that hold it in. So, if you can get those down. Aftermarket speaker wire make a nice difference in sound. Any other ?
