[kdgsv]

Pulseaudio will (without a lot of tuning) exactly do what you don't want. It will resample most stuff in software, while you exactly bought the DAC for exactly this, e.g. outputting the data and letting the DAC convert it to analog. If you bought the DAC to use its spdif - you bought it for the wrong purpose.

Depending on your pulseaudio version, you can set it to 24 bits by default, set 44.1 khz as your default sample rate and 48 khz as your alternate samplerate and additionally enable the setting: avoid-resampling - that way pulseaudio will behave as good as it can for every sort of content - else it will just resample all content, reducing bit-depth and so on.

Hint: As you did not hear it, it's most likely not a problem for you.

I want to have the audio output always on the SPDIF optical output of my motherboard. If possibel bit perfect for listening to music. I've only flac format music.

If Kodi is using HDMI audio output the quality is not as good because it goes through HDMI to the TV and from there to the DAC which I want to avoid.
I assume there is something not perfect configured on my system.

What would be the best approach?