|But you just contradicted yourself: ABX tests tell you what they tell you, but AB tests tell you nothing? (I'm inferring this by your suggestion that they aren't worth anything.)
Experimenter expectancy is so fatal to the reliability of data that yes, there is really nothing to be gained from a sighted test. Yeah, if you do a blind test where you stab someone with a knife, and where you poke them with a bat you're probably going to get the same result, but that doesn't prove anything. There's been so many studies on the effect of experimenter expectancy, and so many scandals from before it was widely observed, that it's pretty ludicrous anyone would trust the results of an AB test they didn't already think they knew the answer to. For example (HARHARHAR) take two knives and tell a person one has been sharpened and is brand new, while the other is old and hasn't been changed in a long time. The knives are actually the same. Do you think there'd be a disproportionate number of people saying the supposedly sharp one was actually better? That's the problem with testing audio equipment you can see. You're influenced by it's aesthetic appeal, what you've read about its brand, and how expensive it is. Tests like that are not legitimate, and I wouldn't spend money depending on their results. It's as good as not doing the AB test in the first place.
An abx test is valuable because, if the results come back positive, we know something for sure! There's detectable difference. If an abx test comes back negative it doesn't conclusively prove that there's no difference to be heard, and it also doesn't prove that my ears won't be better than whoever performed the abx test, but at least it tells me what we don't know. An AB test tells you neither because it can't distinguish between a placebo effect and a real effect as the variables co exist. You have to get rid of variables to know exactly what is affecting what. An AB test is only useful if you're trying to prove something about the person and you already know about the objects they're testing.