I know many people look at frequency response as the end all be all of audio
Tests/measurements should be looked at, as well as listened to, that's how amps are built and designed.
Here in Australia we had a
PrimaLuna DiaLogue Premium HP
owner who was complaining about hardness in the top end.
I showed him and explained the peaking in the top end of the stereophile frequency response and square wave tests, he then put it up for sale on the same forum.
I was castigated by other PL owners for doing this, all they could say was to listen to it, graphs/tests are meaningless, and buried their head in the sand.
I then proceeded to explain how this peaking problem could be fixed, and other techs agreed that the HF peak could be eliminated, with about 1hrs work that involved the feedback network and HF filters.
The ones that castigated me, suddenly wanted to know what and how to do this themselves, but knowing they would be incompetent without the necessary tools, I just gave them names of very capable tube techs that were able to do it for them, because they didn't have the frequency generators and oscilloscopes needed to do the job properly and then to check what was done.
They quickly went back into castigation mode.