If i understand the settings correctly then:
Speaker power level=amp gain. So it works only if the speakers are connected directly to dudu internal amplifier. And then spdid has nothing to do with speakers so it will be logical to turn passthru off.
Spdid passtrough is only needed to be turned on if you use spdid signal from dudu to external dsp and you do not want to use internal dsp. And then the speakers are connected also to external amp.
So i think if you turn spdif off, and can then manipulate power level, it should be okay.