changing the Head unit on almost any car with an aftermarket head unit could be possibly the biggest upgrade that you can do to a sound system. It provides better sound shaping, and allows you to control sound better (ofcourse this matters on what head unit you get. ex. pioneer premiers have alot of sound control). Sound shaping = highs sound more precise(sound like highs) and lows sound better (sounds like an actual low). I'm getting a head unit soon, as I just finished making my sub box, and my car is friggin loud as hell. powering a 15" Kicker L7, with a 1501D MTX amp. I have to leave the gain down to 25%, cause it's to damn loud, my ears hurt right now. But hopefully with a new head unit I know that I will be able to control the sub better, knowing which hertz are sent to the sub so that I can take away the painful noises