I think the problem you might face is the base audio system is so bad, BMW had to do some heavy signal processing to boost bass and highs to over compensate. So when you put in speakers capable of reproducing some of that part of the sound spectrum unaided, you get far too much of it.
As far as I can tell the first thing people do before replacing speakers even if they keep the base amp is to undo the signal processing. Lot more on that in the audio section.
BTW a buddy of mine accidentally ended up with the base audio, truly sucks ass. Shameful in a 50k+ car.