I'm VERY new to the forum here, but I'll put my $0.02 in here...

I swapped head unitis in both my 5-series and 7-series sedans (admittedly, they are older models). In doing so, I ended up having to bypass the factory amplifier and build my own crossovers for the front and rear speakers. I later replaced them with new component speakers that had their own crossovers and added a Rockford Fosgate 4-channel amp, but that may be more than what you're looking for at this point.

What I'm trying to get at is that the factory BMW headunit has a low power output that goes to the amp. Your new headunit has a much higher output that is pretty close to the output of the factory amp.