[D126]

To my knowledge, maxing software is best and using DAC volume control is best.
 
My DAC volume control knob starts at 8 PM and I have it to about 11 PM before it is too loud with Foobar2000 at 100%. 
 
Is this optimal?

 

[Transmaniacon]

This is how I do it, keep the source at 100% and adjust my amplifier gain.  Some people do 75% source volume, but I really don't think you can tell a difference.