"It was probably not measured properly and not really flat. Why else would it sound dreadful?"
Actually, you are right! I was only measuring the on-axis response, and ignoring the off-axis response, because back then I didn’t know any better!
But here is another critical piece of the puzzle: "Flat" sounds "thin and bright" to most people.
And a gently downward-sloping curve sounds "flat" to most people.
There’s an excellent discussion of the subject in this review of the Dutch & Dutch 8c loudspeaker, scroll down about 1/4 of the way:
https://audiophilestyle.com/ca/reviews/dutch-dutch-8c-loudspeaker-review-r739/
The author draws on an extensive controlled blind study conducted by Sean Olive and Floyd Toole, and says,
"Most participants in the study preferred a frequency response from 20 Hz with a straight line to -10 dB at 20 kHz. A measured “flat” in-room frequency response is not the preferred target, as it sounds too thin or lacking bass."
If I was designing a studio monitor the goal posts would be in a different place, and "flat" measured response might very well make sense, depending on what type of monitor it was (tracking, mixing, or mastering).
Duke