Let me share my recent experience. I just bought a 2000 Outback wagon with 85k. Previous owner, at original time of purchase in early 2000, had upgraded the entire sound system with OEM Subaru speakers, tweeters, subwoofer, and 6-disc CD player.
I was not happy with the sound; quite a lot of distortion at modest volume, confined to driver's side, front and rear. I took door panels off and discovered that the speaker paper cones had separated from the metal frames. Clearly a bad batch of glue. I replaced the speakers with the same set from Subaru, and all is now well; sounds good. Lesson - check the speakers for failure before doing anything else.
Now, if only I could get the CD changer to work.