pi3 sd card vs Pi4 wifi or ethernet sq

[the_bertrum]

The only other thing I can think of which could affect sound quality is rf noise and power supply modulation affecting the analogue bit of the D/A process.

Now we are talking.  There is a plausible mechanism there certainly.

I think we still have a lot to learn about digital signals and how to transmit them via cable or wireless without audible damage.

Nope, we know very well how to transmit digital signals without damage. You never send a £50 payment for something only to find the message damaged and it becomes £500, or £55. We send hi-res sounds halfway across the world from TIDAL and never once is the signal damaged.

I have personally verified that it becomes difficult to convey a digital signal for a few tens of meters, with cable, passing through some switches.

Finally, because by replacing the DC power supply of the single switch I get so much difference. On the latter topic I am in tune with "philrandal" which finds the cause in the very high frequency noise that passing through the power supply interferes with the digital data and therefore an excellent power supply with high PSRR eliminates the problem.

So here we begin to see a mechanism that works. If noise is being introduced by a switch and transmitted along with the signal to the DAC, then the noise can interfere with the information in the signal during and after the conversion. In this case there is no need to invoke some hitherto unknown method by which the information in the digital signal is altered in some subtle and non-destructive way simply by travelling through a different medium.