"If you upsample a nonDSD file to DSD512/1024, iDSD first oversamples to 705.4/768kHz. None of the filters mentioned above applies in those 2 modes. You can turn the knob, and may appears that you are switching digital filters, but they are not doing anything."
I don't think this is true. At lest @iFi audio has confirmed that any of the PCM filters can be selected and works if PCM files are used. The DSD upsampling just takes place after the selected filter is applied.

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"If you upsample a nonDSD file to DSD512/1024, iDSD first oversamples to 705.4/768kHz. None of the filters mentioned above applies in those 2 modes. You can turn the knob, and may appears that you are switching digital filters, but they are not doing anything." I don't think this is true. The filters work even if DSD conversion is selected for PCM files. The only case where the filters don't work is where the source file is DSD.

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The DSD512/1025 upsampling overwrites all other filters. See this official iFi post. https://www.head-fi.org/threads/ifi-audio-pro-idsd-the-official-thread.869144/page-19#post-14392333