I have enjoyed crackle-free music in the last couple of days. There is no explanation why, I didn't change anything. However, I have still noticed subtle artifacts all the time. They did not sound like little electric sparkles as before, but more like small recording issues. In other words, the crackling became quieter. They were as if someone in a big orchestra dropped something in the background quietly. Some of these noises are indeed in the recording but some are unfortunately not: when I rewound the track they were not there. I would say, these "artifacts" are tolerable, they don't ruin or distract too much from the music. So, not bad.
Out of curiosity I picked up an inexpensive
Behringer U-Control UCA222 this morning (I had to drive to Germany as such products are not sold in, or delivered to Switzerland which is not part of the EU).
This little thing converts digital USB to optical S/PDIF which I connected to the M Scaler. I have listened to two quiet albums today and so far I could not catch any "artifacts" that were not in the recording. In other words, switching from USB to optical on the M Scaler eliminated even these very subtle noises. Looks like these artifacts didn't come from the mains after all. I have even moved everything back to the original wall socket. (This doesn't mean that the M Scaler could not sound better off grid, of course. I understand from
@Triode User that RFID isn't directly audible.) Looks like my overpowered laptop, a maxed out 2018 MacBook Pro 15" with six-core i9 CPU, is the cause of the crackles getting into the M Scaler via USB (note that I have a JitterBug and an AudioQuest Cinnamon USB cable).
However, not everything is perfect. It turned out that the Behringer UCA222 is limited to 16 bit / 48kHz which is not enough. I could live with a limit of 24 bit / 192kHz alright though.
So obviously I've become a big fan of the optical connection now. This system never sounded so good. Currently I am looking for a Roon endpoint with optical out. Let me know if you know a good one. An older Mac Mini could probably do but I would prefer something that is not obsolete and can be replaced if it dies. (I have also considered Antipodes because they are belived to be very nice even via USB but they are very hard to import into Switzerland and are very expensive, especially with the additional customs fee.)