Please let me show now that even class II components and only one component in class I may not be a cure. These measurements show effect of capacitive currents flowing through an AC/DC wall supply adapter of DacMagic+. The system is compose from a DAC, a preamp, an USB soundcard and a PC. PC (and thus a soundcard) is in class I, all other components in class II.
1) DAC is supplied from its factory supplied AC/DC wall adapter. There is no ohmic groundloop, just the loops through capacitances of power supplies, either SMPS or transformers.
Block diagram of the system with DAC supplied from its wall adapter
Noise spectrum of the system with DAC supplied from its wall adapter
2) an isolation transformer 1:1 is added before the DAC wall adapter. Its primary/secondary capacitance is about 150pF
Block diagram of the system with isolation transformer added
Noise spectrum of the system with isolation transformer added
We can see that the isolation transformer added before the DAC wall adapter improved suppression of mains components considerably. This clearly demonstrates issues of the single-ended signal interconnects in audio. Properly designed balanced link would be a real cure.