Greg, your post inspired me to do a quick comparison test between the 2V and the 6V setting.
My Vivaldi stack output is usually set for 2V going direct to my Amp which has a sensitivity of 1.3V - allows me to listen mostly at “25dB” or better volume setting on the Vivaldi.
Using a Dayton OmniMic, I recorded my Speaker output from the listening position with the Vivaldi set to 2V compared to 6V; Speaker outputs level matched on pink-noise to within 0.2dB, and recording peaks at -9dBFS, tops.
It was tricky to perceive any tangible differences, but analysing the recordings via Adobe Audition, turns out the 2V setting recording has a 3dB better dynamic range than the 6V setting! 
I think all things being equal, clearly it’s best to follow dCS’ advise and set the DAC’s output voltage level to whichever is closest to the Amplifier sensitivity. 