It will depend on the equipment. I guess every combination of amp and source will have it's sweet spot. Just a matter of trying what works best.
I believe some digital sources only have volume control by attenuating the digital signal (by reducing bit depth thus dynamic range). If this is the case having the source at 100% will have an advantage as long as the amp in the source can put up the volume cleanly and the amp can handle the input.
I started a thread about this in the "sound sience" sub forun but no replies to date.
When I run my android tablet through my bravo tube headphone amp it is clear that the combination works best with the source at about 60% even though I suspect the volume control to be in the digital realm (pre-dac).