Curious, but if it works for you its great ! On my system I could hear a degradation as soon as I started using either the Mac volume control or the one in the Spotify app or a combination of both as you seem to do. My DAC doesn't react to the Volumio web interface, its just on 100% all the time. But for casual listening it is more than fine.

There is some interesting info here about the i2s interface and the path the zeros and ones take to get converted into what we love best, beautiful sound.. I think that is part of the "secret" why the Pi/DAC combo sounds so good because it circumvents the usual interfaces (USB, Optical, etc.) and gets the data to the converter on a very short path (no cables either)

https://volumio.org/raspberry-pi-i2s...ounds-so-good/