Those graphs are fantastic!
I just made a grounding cable and gave it a shot. From what I see with my DMM the XLR pin #3 on both channels, the chassis (screws, not the auto power switch), and all RCA sleeve's are tied together as they should be. Now, the interesting part. I audibly get more noise when grounding. I dont hear a 60Hz buzz/hum, only a faint ~500-600Hz sine wave/harmonic when grounded. Again, the gain is so insanely high when this is audible on my stack that I would destroy my ears and IEMS if I played anything through the DAC. If my method of testing for hum and noise floor if flawed, let me know.
The switch mode PS brick is still just a generic throw away. I'm in the continental US with Romex wiring instead of BX/armored cable and I do have everything plugged into a pure sine ups, but I wouldn't expect that to be doing anything special. Maybe I can consider myself lucky, since your graphs indicate a hum of something very audibly noticeable. No wonder you're not much of a fan. I should also mention that I've got a DROP labeled amp instead of the massdrop unit.
What do you think is going on? Different power brick? Design revisions? Unit variation?
Does anyone else notice a 50/60Hz buzz/harmonic on their 789? 887? SP200?
Again, its so kind of you to be looking into this. Thanks again!
Here is a pic of the brick.
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