Digital volume controls work by “discarding” bits.Therefore when set at 0.0 nothing is discarded and you have maximum resolution. However it is unlikely that you would actually be aware of any subjective loss of sound quality until the reduction in level below 0.0 is significant. By then, however, the loss of volume may be such that you would not actually be aware of the quality loss. I have listened and listened to the effect on sound quality that setting the control well below -20dB brings and , frankly, have nothing to report. It just sounded quieter as it should.
As for the voltage settings these are limited in scope on the Rossini and exact matching rather than approximate matching is not possible. The 0.0 figure refers to dBfs ( decibels full scale). That is if the programme material has a section at the maximum recordable level (0.0dBfs) then the Rossini will put out a voltage of 2 or 6 volts , depending upon your voltage selection. Of course the actual voltage level will vary in accordance with the recording level moment to moment and a 2 or 6 volt output may rarely or only briefly be encountered in practice ( partly depending on genre of music).
That figure of 2V and the recommendation to use it is not a random figure but is based on the industry standard for the voltage that CD ( and SACD) players should produce for a recorded level of 0.0dBfs. The majority of modern preamp line inputs are designed with something around that figure in mind for sensitivity. The need for 6V for a line input on a preamp would be, IMO, unusual. It may, however be appropriate for certain tube power amplifiers with comparatively low input sensitivity and where the DAC is directly connected ( no preamp).
Some old preamps, mainly dating from before the CD era, may overload even with a 2V input. dCS does cater for this circumstance ( and for some power amps with very high sensitivity) with e.g. the Vivaldi DAC which also offers outputs of 0.2V and 0.6V for 0.0dBfs recorded level.