If you are going to replace the amp/dsp (it's a combination box), this is what you want:
http://www.bimmer-tech.net/audio-amplifier-for-bmw/. They also supply the correct wiring harness for the E89 Z4 (it is unique to the car, ridiculously, like many other parts that otherwise seem the same to E90 cars).
That amp will not only drive your speaker setup, but it has a preout 2-channel so you can run an external amp/sub if you desire. With built-in DSP and software provided by audiotec-fischer you can completely configure its behavior.
Why do you see other choices as a better alternative? Just curious ....
Filippo