I have a 05MY STi and need to do something about the stereo - is it the stereo which is shyte or is it the speakers? (or maybe it's both!)

If I change the head unit will it sound any better?

Is it quite a simple job to change it for a better one?

 

I have a 06 and change the head unit now it acceptable

 

Stereo changing is simple and easy.
If you are changing stereo try change speakers(speakers make unbelievable difference).
I brought Alpine CDA-9886R with front speakers Alpine SXE 1325S,and sound is amazing.