The merit of abx testing or the validity of what was documented in 2004 is not, I suspect, about there not being differences in power cords but that it shakes the foundation of how we "think" we hear differences. And for that reason alone, there is much to be learned from it.
I would surely like to know if I have the ability to discern such sonic differences, but I suspect I would fair no better than those in 2004. But, stepping into the confessional, I have made purchasing decisions based on listening periosd far less than weeks/months because I was confident that I consistently heard not just differences, but improvements. In the times where I've heard power cords such as the JPS Aluminata or Kaptovator inserted into a system, I was totally convinced I heard a discernable difference (as did most everyone else in the room which could also be a contributing factor). But, if in participating in such an ABX test, I was forced to accept that I am simply not able to reliably tell the difference, it would challenge my assumptions on how I approach power cord evaluation (and likely, most other comparative listening evaluations).
And Werd, strong position aside, maybe that's what you're saying. I do think your premise on how to improve comparative evaluation is valid. Extended listening periods measured in weeks/months in systems and with music you are intimately familiar with should undoubtedly improve your ability to not only hear differences but also to select what you clearly prefer in a repeatable evaluation scenario.
So after sticking with this thread, I'm come to think that quite likely, the real lesson learned from the 2004 abx test was NOT about the cables at all. Rather, if was really just testing the assumption that we believe we can readily and consistently discern such differences.
Entertaining? Sure. But it also tends to shine a bright light on some assumptions that just might make us feel a bit uncomfortable. And that's a good thing.