Where do I set volume control on NAD M51 DAC?

Well, 
At 0db your DAC output level is 2V; at -10dB the output level is 200mV respectively. 
By supplying 200mV instead of 2V you simply void overloading that causes distortion and significant loss of tonal balance and dynamic headroom.

I don't think those figures are correct as the NAD has a different stepping than one might assume. I know the actual top output is 2.375V SE and double that for balanced. -10db is just ten steps from 0db and the DAC can go to -55db before actually truncating a 24-bit file. While I can't say the exact voltage, I am sure the voltage is far closer to 2V at -10db than 200mv. This volume control is from a digital sense 210db in range.

Between the technical measurements and number of user indicated preference, the range that most prefer is between -6db to -10db. If you have a good preamp setup, it won't present any issues. 

I have a C510 which is supposed to be the same circuitry inside. Direct to amplifier it could be harsh & analytical at higher volumes; dull & muted at lower volumes.  Adding a (tube) preamp helped immensely; however as it doesn't have a remote, I find myself using the Nad's rather frequently.  I can go from about -3db to about -20db before noticing any difference and getting up to change the pre's volume manually. I'm sure much of this is system dependent.