Quantifying what one hears is a very subjective thing, but speaking for myself, I have often heard what I would call big differences by swapping one reputable cable for another. Not 1-2%, if the rest of the chain is already very good, and a well-known reference. There's also a learning curve here that needs to be taken into account. People who have spent decades making distinctions about how things sound (I mean live music, not just reproduced music) are generally going to make faster and more reliable distinctions than people who haven't. Even people with lots of experience though need to spend time learning to reliably hear subtle differences between two options - and then deciding which they prefer: interesting, complex music has so many "variables" of its own to take into account. What sounds better partly depends on what kinds of things you listen for and respond to in music.
Of course, the more subtle the differences, the easier it is to hear what you want to hear - whether that has to do with expectation of quality, or brand loyalty, or value for money/affordability (reducing cognitive dissonance around a contemplated or actual purchase), etc. Mood can also greatly affect not just the emotional qualities of music but its perceived sound quality. Plus, every time you listen to a passage of music it's going to sound a little different, if only because you've learned how to listen better from previous exposures. Again, the learning curve comes into play. ABX testing isn't much good for subtle differences in complex presentations, but once you've learned to hear them they can really enhance enjoyment.
Another thing that hasn't been mentioned here is that you have to (try to) control for loudness - the slightly louder always sounds better, other things being more or less equal, that's the physiology of hearing. But this is not easy to control for, can be very insidious if you're not aware of the effect, and complicates judgments if you're not sure whether one presentation is indeed slightly louder than the other, which can be the case in judging headphones - or cables - that have different frequency responses - and every component in a chain IS a filter to some extent. Plus, for this and other reasons, A might sound better at playback level X, B at playback level Y.
Any test that claims to show that there are no audible differences between cables, or that expectation determines what sounds better, would have to address these issues.
One would like to think that the variables in cable construction can be easily quantified and correlated with performance, that they're limited to a few basic interactions internally and with the equipment they're linking, i.e. that it's a relatively simple, well-understood technology. However, this only seems to be true up to a point. If there are a lot of possibly pretentious claims made about expensive cables based on exotic/esoteric science, there's also bad, reductionist science "proving" those claims can't be true. In the end it's better to check things out for yourself and have an open mind - no matter how unreliable it might be, at least it's yours!
Edited by tonereef - 11/12/11 at 1:00am