Wow, exhaustive. I’m running Roon on a Mac Studio, to an Audiobyte Hydra USB interface, a Musical Fidelity DAC, then on to powered monitors. I’ve been learnning more and more about the infinite variables available to us, from your results it sounds like I can run this system without a passive volume control or preamp in the system, but that I can attenuate volume via Roon DSP. That’s kind of incredible!
I wish there was a way to similarly attenuate all audio sourcing from the Mac (Apple Music, streaming, etc) in the same way, or even to run it through Roon MUSE so I could achieve same kind of bit perfect fidelity.