I have a DUDU7 now and will give some information in case somebody else has the same questions.
The TOSLINK output uses 48 kHz sample rate. The output signal is not at full level. With volume at 36 and SPDIF passthrough activated it looks like this:
I used a 0 dB 1 kHz sine signal. I would except that pass-through is really pass-through. The EQ is bypassed but volume control (Balance sound + Main volume) is still in effect.
If I change the setting in "Balance Sound" from 5 to 10 the signal is still not at full level:
USB-Audio works without problems. 96 kHz sample rate is used and the signal is at full level:
So if you want a perfect signal use USB DAC. Of course there will be resampling to 96 kHz.