Where do I set volume control on NAD M51 DAC?

The Nad only puts out 2.375V SE at full output, a little over the Redbook standard and should pose no overload problems for any well designed preamp.
If you do reduce it, try to stay at 75% or higher, as some digital domain volume control can "bit strip" if below 75% of full output. EG: 14bit resolution or 12bit and further if it’s very low.
Thorsten Loschech of AMR, says he can hear a detriment of just a small -10% reduction from full up, on digital domain volume controls. I can’t, it needs to be -25% or more for me to hear it.

Cheers George

While true that digital volume controls can truncate the dynamics in music when applied to significant degrees, the NAD M51 has a 35-bit over the more typical 32-bit digital volume control implementations, giving it a bit more room before it begins to truncate the digital signal amplitude.

During the measurements via Stereophile, the small amount of overdrive that won't be any issue for nearly any premap, had increased the noise floor of the DAC notably. When lowered to -10Db, it was able to achieve 21-bits of genuine resolution across the audio band. I still believe that M51 is a benchmark for digital volume control implementations, I would agree that too much digital attenuation will prove detrimental to the audio signal.

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!