Hello TorstenW301
I have the SPH-20DAB connected to a power amplifier. This is also from Pioneer and it works wonderfully. Important: The power amplifier must be able to process a "regular" output (i.e. the output that would actually already be for speakers). So you have to make sure that the amplifier (= power amplifier) digests the output power of the radio as input power.

Does the de-amplifier have power?
Does it work with another radio?
Have you hooked everything up correctly?