If the sound system is now head unit straight to speakers, adding an amp will be pretty much a standard install.

As mentioned IIRC the speakers of the DSP system are not standard impedance...but if a head unit can drive them any amp should be able to.

Sub I am not sure, need to check impedance on those and see if the amp you are thinking of can drive them.

If I'd gone as far as replacing the head unit I'd be seriously looking at changing the speakers too if I added an amp.

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04 X5 3.0i auto
03 X5 4.6is