At what volume can you operate a NuP amp in an optimal manner - high volume and low distortion. I am guessing the distortion numbers given in the specs are at 100% volume?
I might add, that NuPRIME DAC's use an "Advanced, FPGA implemented thin-film switched-resistor ladder network for controlling volume, with a single resistor in the signal path at any volume setting".
So what this means, is you will experience the least distortion and information loss of virtually ANY volume control.
Maximal Listening volumes then will be determined, by your amp and speakers, and the SPL/Range, they play best in.
This means for practical purposes, the Volume Control will not introduce sonic issues, but the limits will be the Amp, Speakers, and Room Interactions (if applicable)