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PCM uses the narrowest alternate the DAC advertises that can hold the file. On FiiO KA17 a 16-bit file opens the 16-bit alternate. On SMSL with Thesycon firmware, 16-bit and 24-bit open the 24-bit alternate, and 32-bit opens the 32-bit one. One advertised alternate carries every PCM file.
ampdac show transport and ampdac show status list the advertised alternates and mark the one that is open. The README states this rule.
The Volumio package also keeps the kernel DAC in the output list and restores its hardware volume when playback leaves libampdac.
Install Volumio from ampdac-volumio_0.3.21-2. Revision 1 stopped during setup: editing the MPD template on the /volumio mount failed with "Device or resource busy". Revision 2 writes that file in place. FiiO KA13 volume buttons keep one level on the kernel driver and on libampdac. Info names the DAC that is plugged in.
Install and remove commands are in the README.