There is an extensive thread on speaker replacements and results people have experienced.
I love the sound results of the speaker changes that I made as a result of that thread.
Search is your friend.......
If I can find it quickly I'll post it.
Edit: Check this out. I think it is a very thorough vetting of the speaker upgrade options for our E53, just MHO:
https://xoutpost.com/bmw-sav-forums/...-speakers.html
Mike