>>>"there is no scientific reason why a power cord would change or improve the soundwaves comming from your speakers."<<<
That is simply not true. There are easy to understand scientific reasons why aftermarket and stock power cords affect sound and performance in most systems.
You may be right that component upgrades are more worthy than a power cord upgrade, it really depends on the system and context.
Double blind proof? In most cases the differences in comparing aftermarket power cords are so obvious that would not be necessary. However for the unconviceable--and those that have never run their own simple comparisons, even governmentally notarized triple blind proof would only spur more argument and cries of "prove it!" and "it's all a scam". In reality, there is no such thing as "adequate proof" for those that initiate and prolong argument on these points in chat forums.
This comes up less often than it used to, which shows that more people understand the pointlessness of on-line debate over such matters.
Placebo effect _does_ exist, however IMO it falls somewhere short of extreme self delusion and ignorance of the obvious, at least for those that have taken the time to perform their own simple tests.