i have a new shape '04 audi a3 sport, a standard audi concert head unit with front speakers, rear speakers and a sub in the boot. i want to upgrade this system to add an amp and better sub & speakers. how do i do this? how do i add an amp to the concert head unit? what size sub can i put in the boot? what size are the speakers? i dont want to spend a lot, but still want power & quality to pump the music with the windows down.