I haven't read this whole thread. I don't know if this has been mentioned yet, and I'm not arguing about the USB signal being affected by cable quality, although if it were bad enough perhaps enough packets would be dropped to make a difference, but It would have to be a damaged cable, or have some fairly intense interference as USB has very aggressive error checking, but anyway, I have found a situation where USB cable quality affected sound quality greatly. Using my R24 Recorder/Interface/Controller/Sampler/Doohicky it has three power sources, battery power, wall wart, or when it's plugged in to a USB source and switched off it powers itself from USB. USB has four wires two signal wires +5v, and Gnd. With the R24 using the USB cable for it's ground I had one cable when being routed through the mess of cables behind my computer would pickup interference from my cable TV and hiss, The same cable was fine if I turned the R24 on before putting it into Interface mode because it would instead us it's wall wart for power, but I swapped the cable out anyways and now it's fine using USB power.
I wonder if any of these other DAC's are using the USB provided power or ground, although I'm sure that the R24 doesn't have the best power supply either and probably isn't doing very much filtering as all of the power that gets to it is supposed to be DC +5v.