I have huge respect for your technical expertise, but sorry, if you are not a violinist or any other trained classical instrumentalist you may not realize that close exposure to real instruments teaches that these natural sounds are BRIGHT (in the good natural sense, not for example in the artificial types of distortion that is obvious if you tune the radio slightly off its freq and get the static which brightens the sound).@viber6 I agree! I've been playing instruments since I was three years old (piano); started harpsichord when I was in 6th grade. I picked up string bass in 7th grade and played in orchestras and ensembles well after college. These days I'm in a rock band playing keyboards again only now they are synths and a Mellotron. But I also play flute and have 2 albums of that; used to regularly play out until the pandemic.
The brightness I'm referring to isn't the correct natural brightness of instruments; its caused by higher ordered harmonic distortion generated by all electronics. Your radio tuning example is a great way to illustrate how this works. The ear interprets all harmonic information as a tonality (this is how we can tell the difference between wound and gut strings for example). Traditional solid state amps and tube amps with feedback sound bright because the distortion I'm referring to isn't masked, so the ear interprets it as brightness- quite independently of the instruments being portrayed in the recording being played back. This is why two amps can measure perfectly flat on the test bench but one will be bright while the other is not. Put another way, this kind of brightness is not a frequency response error.