Milpai, as humans we hear in a very specific way- there are certain types of distortions that our ears don't care a lot about and there are other distortions that our ears care *a lot* about.
For example, we used the presence of odd-ordered harmonics, specifically the 5th, 7th and 9th as a means to determine how loud a sound is. This is arguably one of the most important rules of human hearing as being able to tell how loud a sound is is the sort of thing that can say whether you live or die!
It happens that tubes can make less of these harmonics than transistors usually can. There are exceptions (although they are few). To do this one would have to avoid the use of global negative feedback as a design element regardless of the circuit being tube or transistor. This is a lot harder to do with transistors (since they are not as linear) than it is with tubes (triodes are the most linear form of amplifying device known).
It turns out that global negative feedback, which is normally used to great success in reducing overall distortion (THD) actually enhances, to a very small degree, the odd-ordered harmonics that I mentioned before. Our ears are extrememly sensitive to this enhancement; 100th of a percent is easily audible and audiophiles have terms for this that you have heard before: sheen, hard, harsh, brittle, clinical, etc., all expressions of a very small amount of odd-ordered harmonic enhancement.
For example, we used the presence of odd-ordered harmonics, specifically the 5th, 7th and 9th as a means to determine how loud a sound is. This is arguably one of the most important rules of human hearing as being able to tell how loud a sound is is the sort of thing that can say whether you live or die!
It happens that tubes can make less of these harmonics than transistors usually can. There are exceptions (although they are few). To do this one would have to avoid the use of global negative feedback as a design element regardless of the circuit being tube or transistor. This is a lot harder to do with transistors (since they are not as linear) than it is with tubes (triodes are the most linear form of amplifying device known).
It turns out that global negative feedback, which is normally used to great success in reducing overall distortion (THD) actually enhances, to a very small degree, the odd-ordered harmonics that I mentioned before. Our ears are extrememly sensitive to this enhancement; 100th of a percent is easily audible and audiophiles have terms for this that you have heard before: sheen, hard, harsh, brittle, clinical, etc., all expressions of a very small amount of odd-ordered harmonic enhancement.