I did some power supply measurements specifically on the RPi Gen 4. They are posted elsewhere on the forum. Take a look if you are interested.
Bottom line: with most endpoints and computing devices like NUC’s, your linear power supply is directly feeding an SMPS! Audiophile dammit!
Off line 120-240 Vac to 5 Vdc or 12 Vdc, etc. SMPS having two-wire mains plugs do dump line current into their grounds and outputs. This ends up on your system ground.
All you have to do to solve this particular issue is run a ground wire from your SMPS (-) output to earth ground on your mains outlet.
Done. No need to worry about ground loops if your DAC is properly designed with transformer or optical coupling.
Far be it for me to say that using a linear supply won’t make a difference in what you hear. It’s your money, your system, your unbridled listening pleasure.
It’s totally ok that power supply differences in these systems may exist solely in your auditory cortex. That does not make them any less real.