I agree with the above post.
I removed the cassette player in my '87 325 to put in a new head unit with a cd player. The crackle went away and the stock speakers really aren't all that bad.
You can't turn them up enough to deafen yourself without blowing them out probably, but IMO, they're okay. The premium sound system sounds betters. The door panel tweeters help a lot w/ making the sound come from the front and the overall sound from the rear premiums is better too. (The rear deck for the standard/premium speakers is different. They'll still fit but just FYI)