A ground loop may affect both the noise floor and harmonics. There is a similar test with RME audio interfaces and an Intona isolator:
https://intona.eu/en/support/answer/1261.
The link was not working for me.
Is the Isolator compatible to RME devices?
I have been using various laptops as my source to stream music over USB that I listen to upstairs in an office and in the basement in my game lounge. In the office is an RME ADI-2 DAC FS (2018 model) and in the lounge I have a Topping D30 Pro. Both DACs use simultaneous outputs connecting the balanced outputs to an amp with the RCA output to a powered subwoofer.
Before I added a subwoofer to both rooms, I noticed that I could hear noise with the RME DAC (via RME's 1/4" Phones jack on the front) on the low-power setting when I was listening to my relatively sensitive/efficient Denon D5000 headphones. I'm talking barely perceptible noise in quiet parts of the music with the volume cranked up higher than I would typically listen to for any extended period of time; but, I could hear it.
From my research, and recommendations from RME, I purchased an Intona USB isolator. That device perfectly resolved my issue.
In the game lounge with the Topping DAC, when I added a subwoofer to the RCA outputs, immediately I was hearing a noticeable 60Hz ground loop hum. Again, the Intona resolved this issue completely. I purchased another Intona USB isolator so I did not have to drag it around with me between both locations, and I am quite satisfied with this solution.
I got the industrial versions of the Intona, so I expect them to last longer than I will be around. These devices allow for me to use practically any laptop or computer as a source, without any issues from ground loop noise.
I will say that I was surprised that my HP Windows 11 (upgraded from Windows 10) laptop is much quieter than my Apple MacBook Pro M1 with regards to ground loop noise from the power supply. The MacBook costs twice as much as the HP laptop.
Anyway, the gist of my post is that a good USB isolator can be found that should help with ground loop noise without impacting sound quality in any negative way. With these devices, it allows for anyone to use existing or otherwise inexpensive computers as a source over USB. It seems like this HS-01 would have served my purposes adequately at a much more affordable price. (this is where I imagine the song, "Mexican Shuffle" from
Herb Alpert & The Tijuana Brass, would queue up as I make my exit)