looking for speakers for my wrx

While the 08 wrx speakers are way better than the 02’s they still leave much to be desired.

I have no idea when it comes to car audio…basicaly all I want is better sound and I want speakers that will fit right in the stock speaker location.

I’d like to keep the stock headunit…is this possible or will I need an aftermarket headunit?
Also what brands should I look for and what else (power ranges ect) should I look for… Like I said I am a huge rookie in car audio…I have no idea what I am looking at. So any help is appreciated!