USB cables and Galvanic Isolation

USB of newer DACs is asynchronous, meaning incoming data clock and D/A conversion clock are independent.  Sound cannot be affected by the timing of the data, as it is with S/Pdif,  but still can be affected by injected electrical noise (that affect internal DACs clocking).  To limit amount of this electrical noise,  cables with data lines only (no power) are better, when power is not required (DAC has own supply).  Good shielding also plays role to limit ambient electrical noise pick-up.  Galvanic isolation separates two circuits electrically to prevent current flow between them.  It can be done with transformers or optocouplers, but in either case it is not perfect, since there is always very small capacitance between two sides of galvanic isolation, allowing very high frequencies to pass thru.  Nothing is perfect, but galvanic isolation eliminates most of electrical noise.  Another good step would be to reduce noise on the source (computer) side.  That might be difficult since some computers have horrible power supplies.

Yes, electrical noise can enter the DAC but is that really relevant if the DAC has a measure of -120 SINAD from the analog output? Whatever noise came in has been reduced in the DAC to inaudible levels. It might not be perfect but it’s good enough for human hearing.

Amps will have more distortion than DACs unless the DAC is junk and speakers more than amps so if the DAC does a decent job it's not that much of a problem in the system playback. I would say nowadays it's the least of worries. 

One reason I brought up the question of galvanic isolation is that I recently watched a video review of the Chord Qutest by an Australian fella on YouTube.  He claimed to have spoken to the Chord owner or developer (no sure which).  Anyhow, it was emphasized that just a certified usb 2.0 cable be used and NOT an audiophile cable which may be detrimental to the sound.  Something in the Qutest design that doesn't like fancy cables I guess.  I don't have the link but it should be easy to find if anyone's interested. 
That is what got me thinking about GI and cables. 

Some audiophile cables don't meet the USB 2 certification or haven't  bothered to get them certified.