Have you tried any other streamers like SOTM 200 ultra?
I guess for me, going from my gaming PC direct to my dac vs any kind of streamer would make more of a difference because a mac mini is likely inherently much less noisy than a gaming pc.
But interesting findings indeed. Im actually not all that surprised by your findings with the sbooster. Looking under the hood it seems the u1 mini has a pretty solid isolated SMPS.