If a USB cable doesn't work, and you swap it out for a cable in proper condition and that meets the specifications, then the change won't be subtle---that's what we are trying to tell you.
When you have a sophisticated asynchronous DAC (for example the Gungnir) that uses internal high-precision clocks to time the conversion of the digital data to analog data, then there is nothing in the USB cable that can affect any part of the DAC.
You shouldn't put "theory" in quotes---it's not theory, it's how the DACs are actually built. Anyone who has ever built any sort of sequential logic circuit knows this. It's not complicated and there are non-technically savvy folks that can get Arduinos to communicate with digital circuits via USB,rs232,spi,i2c,UART,etc... and this involves the same principles. Follow the protocol---either the data gets there or it doesn't.
The USB controller is a state machine---this means that its functioning depends only on the the 1's and 0's it receives. If the USB cable botches the 1's and 0's, then it will have affected the audio. When the USB cable, USB controller, and computer's USB ports are all operating within specification, the 1's and 0's all arrive as they are sent. Once the data arrives at the USB controller, the USB cable is no long part of the digital-to-analog conversion process.
Nobody argues that your $100+ USB cable works flawlessly---it absolutely should for that much money. The point is, there are plenty of less flashy-looking-buzzword-claiming USB cables available that operate equally as well. The data either arrives at the DAC's end or it doesn't. If the cable is within specification, then it works. If something still isn't working correctly when the USB cable is within spec, then the USB cable isn't the problem and the funds should be diverted to investing in a new DAC, a functioning USB PCI card, or anything else.
If it helps you sleep better at night, you can always invest in some of these and make your run-of-the-mill generic USB cable seem a lot more impressive!