Placebo effect has nothing to do with being dumb. You could be the most brilliant person in the world and still "suffer" from the effect.
I'll admit I am not an electrical engineer (but a computer engineer), so I can't say very specifically how DACs are designed internally. However I do know that there is literally no way a digital audio signal itself can be modified in sound due to the digital cable. At most, you'll have data loss and skipping of samples, which could cause stuttering or skipping. To modify the sound, the sound waveform itself would need to change in a very specific way. Again, skipping samples would cause skipping sounds, not a "warmer sound" or "better soundstage". It's just impossible.
Do modern DACs actually run directly off the USB data feed without any buffering? Do they not have their own local clock signal? If they run off a USB clock and/or don't buffer the data feed, that sounds incredibly stupid to me. That's not a digital problem, it's a flaw with the DAC circuit implementation if clock jitter is permitted to be introduced to the analog output. Still, this won't change the sound signature. Even in this case a better cable won't give you boosted bass, or "better soundstage" or whatever; at most, a jittered clock to the DAC could definitely make it sound distorted and "rough" sound, but again, this is a flaw with your DAC circuit implementation. And again, even in this case, there's no way it can give you a "bass boost" or "louder treble" etc.
This was my only point to begin with. People who imagine a bass or treble boost or totally different sound signature coloration must be imagining things, because this is simply not possible. Jitter and noise coming out of the DAC are of course to be expected, but that's a DAC issue. The cable shouldn't effect this, if the DAC circuit is properly designed.
Edited by ac500 - 8/1/11 at 4:57pm