The speakers in your link don't appear to be oem replacements. I say that because every Bimmer I've owned has a three way component system upfront and a two way seperate out back. That link offers two way seperates.
If you wanna keep everything oem, buy new speakers at the dealer. Aftermarket speakers are better, but exhibit significantly better build quality. This added beefiness requires more power than do oem speakers. Therefore aftermarket speakers used with an oem amp and head unit would yield a loss in amplitude.
From your description, your problems sound representative of an old used up speaker (not your amp or head unit). While it's possible a channel of your oem amp is going, it's not likely.
If you have any plans to add an aftermarket head unit (doesn't sound like you do), replace with nice aftermarket speakers, if not, go with oem.
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