[Jay-ZS+]

There are so many things that will effect the sound quality from the sound file, quality of media cd/mp3, headunit,amps,installation right down to the quality of RCA and speaker cables you are using. The Headunit is the inital part of the system with a top quality HU you will start off with the best possible quality but start off with a budget unit and the SQ will be instantly be lower and no amount of gold plated cables, high quality amps, silk coned speakers will make up for that intial loss. As James said you cant go off of brand name alone as all the 'household' brands make both high quality units as well as the budget ones often found half price in halfords! lol

Obviously the change of speakers alone will make the largest noticeable difference, thinks like how they are installed make just as much difference. One of the reasons id always recommend some sort of soundproofing, the more the better, just 'bounces' the bass back into the car rather than sending the soundwaves rattling though the body pannels!!! Theres often a massive difference between a loud system and a sound quality system.