Doesn't really seem like the head unit should make a difference... if anything they should sound better. I'm not sure about the speaker sizes, but you can find them at www.crutchfield.com

As a side note, I can tell you why the rear speakers in my legacy sound muffled. It's because I don't have any! My car came from the factory with two lonely speakers. I've gotta do something about that...

 

every speaker in that car is a 6 1/2... also, Do you have your factory speakers amplified??? if so, check the cross over on your amp... probally have the wrong setting. Also if you are thinking of replacing the speakers, go with the new infinities... i just got them for x mas and they are sooooo nice.. the tweeters from the component set fit under the stock tweeter grills and the sound is just right. I have mine amplified but they sound great either way. If you got any more stereo questions ask me as I have gone throught that whole car and replaced everything that has to do with audio.