@mrskeptic , that statement is based on the research paper I posted on page 1:
Flat on-axis frequency response is clearly the engineering objective for most of these systems. Those that deviate significantly earn lower ratings in double-blind subjective evaluations.
And in section 3.6
Double-blind subjective evaluations of loudspeakers conducted by the author and his colleagues for 35 years have shown consistent preference for those having flat and smooth on-axis frequency response, accompanied by wellbehaved off-axis response—i.e., a smoothly changing or constant directivity index ([1] chapter 18)