RME recommend adding these xlr attenuator which are selectable from 10-30db. JTS MA-123. Just add them to the output xlr on your dac. I use them with my Audiophonics Hypex and can keep the volume at around -20 to -10 for music. Other brands are available.
The JTS are excellent, and more expensive "audiophile" ones are probably unnecessary, as the resistors in the JTS seem to be very closely matched and non inductive. Of course feel free to make your own using Duelund CAST resistors - as far as boutique stuff goes they are not outrageously expensive. But I have used JTS ones and I cannot really say I have ever detected any degradation in audible sound quality.