Here's an answer for the tube question:
Our ears have evolved a certain way over literally millions of years. We cannot change that. Tubes and transistors have distortions that are unique to those devices. It just so happens that tubes have distortions that are innocuous to the human ear, IOW they more closely observe the rules that the ear has evolved to. Transistors are less observant of those same rules and so while they have less *measurable* distortion, the distortions that they do have are far more irritating and audible than those of tubes.
In short, if we can reduce the distortions that tubes have then we will be really getting somewhere. Fortunately that is happening in the world of high end audio.
Our ears have evolved a certain way over literally millions of years. We cannot change that. Tubes and transistors have distortions that are unique to those devices. It just so happens that tubes have distortions that are innocuous to the human ear, IOW they more closely observe the rules that the ear has evolved to. Transistors are less observant of those same rules and so while they have less *measurable* distortion, the distortions that they do have are far more irritating and audible than those of tubes.
In short, if we can reduce the distortions that tubes have then we will be really getting somewhere. Fortunately that is happening in the world of high end audio.