anyone installed one? How much of a difference to sound quality does it make? I don't really have a problem with volume, just the distortion and general lack of sound quality. I really want to keep it looking stock so new HU is out of the question, but what should i do to upgrade the sound just to get it up to decent standards? (Sub may be in the picture at a later stage but before that's an option the rest of the system has to sound half decent) cheers!