you dont have to change the head unit. but that would only make them sound better
you can pretty well get even the cheapest brand name speakers and you will get 50-80% better sound.
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but keep in mind. if you want 3 ways , 2 ways. are you going to put subs in at any time? do you want bass up front or just in the back ones?
id used to install /sell car audio and when people walked in and asked what are good ones i pointed them to the sound booth .. go and try all the sets one at a time. because what i like you mite not.
myself i go with good vocals in the front and something with a bit of bass in the back. altho if you have subs on all the time you dont need bass on the back. get more vocals for an all around better sound in total