THD, noise, phase, FR, IMD, clipping levels .... we end up with too many factors to really know what is what. and even then, measurements are made from simple samples, with music it's bound to be worst.
and they all change depending on what you feed the amp with, what headphone or speaker is plugged, and also how loud you're listening.
add to all this a human weirdo and only a blind test can really tell anything meaningfull for ourself. unless you can do measurements on your own system. I don't have a clue how to do that and what I would need to do so(also it's not like I have hundreds of amps at home, I have 2 and they sound identical). but blind tests and AB tests are only if people actually want to know, headfi not being the number one cheerleader for proper comparisons(to say the least) and still very much popular.
happiness over truth, hard choice when music is first and foremost for fun. I know I would take girls and money, but there is no pill for that.
also I suspect tube amp manufacturers to often work toward the "tube sound" on purpose even if that means lesser fidelity. because someone will buy a tube amp expecting to hear a tube amp.