RME have their own forum for ADI-2 topics, perhaps that is best for these issues. Also, their guide is very good and available for PDF download. 17.2 DSD Direct is the section you are looking for, and it mentions "After activating DSD direct in the ADI-2 Dac’s menu… the analog signal is available only at the rear outputs, with a coarse volume control… Output P[h]ones and IEM deactivated. So you may want to use a separate phones amp for that pure DSD mode.