I use Amarra SQ+ on my Macbook to stream Spotify to my Benchmark DAC3 with volume control. SQ+ is an audio driver and intercepts all music sources and streams bypassing the OSX core audio. This is how they recommend using it:
So I have SQ+ and Spotify at full gain and use the DAC3 for volume control.
We would always recommend setting volume control at it's highest level for all apps and devices except for the last in the chain. Use that volume control to adjust listening level. This will make sure that each car in the train is delivering it's best signal to the next car in the train.
So I have SQ+ and Spotify at full gain and use the DAC3 for volume control.