Depends on the engineering- you'd have thought optimal for a DAC would be at full output by design. However, see the Sound BlasterX g6 that s**ts itself above -2db.
That rare case aside, I would have thought keeping it above 70% or so shouldnt have meaningful variance in performance of what comes out. 25/40 (63%) might start going the other way, any noise starting to dominate any distortion. But we are talking negligible of anything compared to what yur speakers/ headphones will be doing to the signal.
I dont recall
@amirm showing what the SINAD was above vol 34 on XLR (4.3v) - would be interested if it went up or down on the V1.