Hi Bob,
Actually, in the past I have tried listening to analogue cables plugged in both ways. It's this kind of behavior that makes my non-audiophile friends question my sanity :)
In some rare cases (like some van den Hul cables that were shielded in the manner described in my earlier post) I did notice a qualitative difference when swapping ends. In general, I found a subtilely lower noise floor and a resultant improvement in clarity one way versus the other, which I speculate is due to optimizing the grounding of the IC's shield relative to the source and destination components. But this was in a certain case that concerned cables with specific characteristics: they were not only shielded, but also had an asymmetrical construction from end to end.
I can't say I've ever heard a significant or demonstrably repeatable difference with non-shielded cables, although I don't pretend to have done any serious experimentation or have extensive experience in this regard. In the end, I guess I've resorted to trusting my ears that the directionality of cables in these cases was of no audible significance (and yes, I acknowledge the limitations of my own hearing and my system).
Please note that I'm not disputing anybody who says that they can hear a difference. I'm just saying that I couldn't in many cases, and particularly in the cases of the non-shielded designs I've tried. This is one of the reasons why I'm yearning for a "plausible explanation" of the phenomenon, to see if there's an objective rationalization that can cut through the "he said, she said" anecdotes. I do thank you for offering Duncan's diode theory, which is interesting to mull over. However, taking the devil's advocate stance, I have to express some skepticism that most (dare I say "any") manufacturers whose cables carry directional arrows have done any of the following:
1. Tested their raw conductors for diotic characteristics.
2. Ensured that the orientation of all the internal conductors used in their cables are aligned with respect to their diotic properties.
3. Tested the diotic effect of their finished cables before applying the directional arrows.