I've got a MY03 WRX and looking at replacing the crap head unit and speakers. Don't want anything to over the top but looking at replacing the head unit with an Alpine unit and sticking in some decent full range speakers.
Has anyone looked into replacing the rear 4 inch speakers with bass speakers, would this be possible with good sound.
What would be the best system to go for? As I don't want to start putting in amplifiers!

 

If you don't wanna use an amp, get a set of speakers to match the RMS of the Alpine HU. (prob. about 25-30W) and get an active sub.

Disconnect the rears.

 

Biggest difference from a single change: head unit
Next biggest difference: front speakers

From thereon, adding a sub and amping the fronts at the same time makes a massive difference again. Beyond that, you're well into diminishing returns for your money but it hasn't stopped me from messing

Don't bother with rear speakers. They're not quite 4", and are more hassle than they're worth.