Where do I set volume control on NAD M51 DAC?

All--
The NAD M51 DAC has a variable volume control.  I initially thought that I should just set it at 0 dB, but then I questioned that.  I can't see where NAD makes any recommendation as to where it should be set.  

It seems to my untrained ear that something south of 0 dB is better than 0. 

Any input on where to set it and why would be greatly appreciated!