The problem I have with a wholehearted reliance on DBT, is that blind testing is useful for picking out obvious differences, but much less useful for picking out subtle changes. A subtle change (like more air in the highs, more "liquid" or smooth midrange, or slightly more coherent bass) are much harder to pick out using DBT, and aren't necessarily noticeable on every track. These changes may be difficult to pick out jumping back and forth in a DBT, and could result in failure on a DBT, even though there is a subtle difference that can be heard. I know when I did DBT testing comparing my ARC Ref 3 preamp to my Sanders Sound preamp, the only real obvious difference I could detect was a slight edginess to the highs with the Sanders pre. This was not always obvious and not on every track. Overall, I was able to pick out the Sanders amp's edginess enough to convince myself of the difference, but I certainly wasn't able to pick it out correctly every single time. Thus, I became convinced that while DBT is useful, it is not the end-all be-all that snyderkv and others seem to think it is.
A lot depends on the ability of the listener to pick out the differences. Snyderkv states the question as whether they make a "humanly audible" difference. Yet, every human hears differently and has different abilities to catch subtle changes. Kind of like wine tasting. Is there a "humanly noticeable" difference between a cheap Cabernet and an expensive Cabernet? To my tastes, probably not. But to a serious wine connoisseur the difference would probably be very obvious. So the DBT is immediately suspect depending on the abilities of the person doing the testing.
The other thing, which has been discussed repeatedly, is that DBT often fails to pick up subtle changes that can be heard over a longer period of time with a component in the system. Switching back and forth trying to pick up a specific change is very different than listening over a period of weeks and a diversity of music sources to get a feel for how the system performs, and then switching out the component and doing the same.
No question that there is a lot of hype and misinformation in this industry, and lots of components are way overpriced for the benefit they give, but I really don't think you can say that for something as subjective as music listening that DBT is the only reliable way to decide whether a component makes a positive difference in the sound or not.
As another example, look at the difference between standard def DVDs and high def blurays. On some movies, the difference is night and day and could easily be picked out with DBT. But with other discs, the difference is very subtle and not necessarily obvious enough to pick out with DBT. So you could pass or fail a DBT on the differences between DVD and Bluray simply because of the discs you chose as your demo material. I would say the same is true with DBT in the audio world, but the differences to be heard are even more nuanced than with video.
The point is that just because you failed a DBT, does not mean that there wasn't an audible difference. It means that the difference was subtle enough that you weren't able to pick it out in every instance or on every track. One has to decide on their own if such a subtle difference is worth the cost of the component in question, but obviously for many people with high end systems, any perceived improvement is worth spending a few more dollars on.
