Where do I set volume control on NAD M51 DAC?

Wow!  The knowledge base on this forum is amazing. Thank you!

-10dB sounds about right to my ear.  Settings closer to 0 dB seem a tad forceful--like volume wars stuff. 

It depends on sensitivity of your preamp.
The idea is to fill up the cup so you can bring it to the table without spilling :)

I have a Primaluna integrated with the following input specs: 

Input Sensitivity: 320mV 
Input Impedance 100k Ohm

Does that help at all?  Thanks Czarivey!

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.