A bad (not to spec, defective) cable can cause trouble even in digital paths, but a solid cable will be no different from a really expensive one. Bits are, in fact bits unless there is a serious defect (impedance, shielding, etc.)
Now, before people pop, there ARE issues int he digital path. At the DAC chip itself (not the box) you must have timing. I explain this int he sonogyresearch blog -- but that only applies to SPDIF, not USB which is asynchronous. A much bigger issue is ground and power noise, which is why sources, LPSs etc can affect sound. I power all my sources with a custom low noise LPS, (linear power supply) and wherever possible I also use a bridge (e.g.: dedicated ethernet --> USB box) to further isolate the DAC from the noise of a computer. All streamers are computers, although they may be embedded in a fancy box.
You would not believe how much testing i have done on these various issues, both as me and through the company. Many things surprise me and do matter. Digital cables, past the point of engineering correctness, are not one of them.
Beware the placebo effect. One thing i can say, for sure, is every system sounds better with a glass of good wine. really. And that tells you a a lot. The wine (etc) variable must be accounted for in real listening if apples are to be compared to apples, which, when in this world, is my business.
