There is imo no need to upgrade those rear speakers I would concentrate your efforts on getting some decent components up front and a sub in the rear. Sound wise the ears hear best from the front due to the fact thats the way they face, there is a huge debate about whether to bother with 6x9s too because sound wise you cant really tell the difference, however, then you have to think about surround sound, the ears can hear 360 but obviously the strongest sound comes form the front which is why you should concentrate on components. Having ran both 6x9s previously and now not running anything apart form a sub in the back, i cant really tell the difference and I have very good hearing being a musician.