I have an M51 in one of my systems and it is a great DAC IMO.
I set it at -20db or so (I do this also with another DAC in my big rig.) Both DACs claim no loss of data at that setting.
For me, it is about who provides the gain and how much. It sounds better to me to set the DAC at a lower volume and up the preamp volume to get to the desired listening level. My preamps are both CJ (ET3se and ET5) and are pretty quiet up to about 85 vol setting, which is way louder than I want even with the DACs set to -20.
I've experimented with different settings for the DACs and the pres. I've even run the DACs down to the equivalent -50 on the M51. I don't usually run the DACs down this low, given my typical listening habits...but of course I had a lot of OCD fun playing with the various knobs and dials.
I've heard recos to have the gain from the DAC at 0 and use lower volume settings on the pres...less noise I'm told. But on my system, this sounds unnecessarily thin and bright, tho' very dynamic. With the DACs dialed down and the preamps up, it sounds more coherent and organic to me with no loss (that I can tell) of clarity, bass impact, etc.
I've also tried the DACs as digital preamps without the CJs in the system. Very dynamic, but it doesn't sound as coherent and organic as with the pres in the systems.
Of course, YMMV.