@hilde45, being involved in the hi fi industry business as well as the recording,
The term High Fidelity from my perspective means being faithful to the original recording i.e. being able to reproduce it as accurately as possible. I have to say that in the thousands of recordings in my collection there are actually very few that sound "bad". By bad I mean nasty peaky treble or obvious distortion. However, if that is what on the recording, so be it. I also have to say that in my experience of listening to quite a few audio systems, very high quality systems make less good recordings more listenable than vice versa because they present more of the music and make it easier to listen through poor recording quality.
The question of people using equalisation etc as presented in your post is complex as there could be subtractive or additive elements involved. For example if an amplifier with high levels of harmonic distortion was used, it will add harmonics to the final signal that are not present (or certainly not to the same degree) in the recording. On the other hand if someone uses a tone control to dial down a peak in the frequency range they are removing part of the signal. Or they could be using a loudness control to boost low frequencies. In that case they are not adding information, but they are making part of the signal more prominent than it is on the actual recording.
Finally, an important factor that bears mentioning is that, at the risk of stating the obvious, people's perceptions differ. Some people aren't that sensitive to frequency response errors for example and other's aren't that sensitive to timing errors. And sometimes, people's sonic values are just plain different, which is why there is such diversity in hi fi equipment.