Problem with "measurements" is that none of us has the same "receptors," i.e., we're all born with, and over the course of a lifetime unquestionably develop, different response curves of our own - the result of genetics and varying degrees of sound exposure and wear and tear over the course of a lifetime. What looks "flat" on a response curve may sound dull, neutral or bright to a particular listener. More important, then, is rendering output across the speakers response curve with a minimum of distortion. Beyond that, it truly is a matter of "buy what you like." Short of that, I suppose, is to listen only to live performance - no question that is realistic and "flat", although even that is open to question these days, as one rarely hears a concert that is not amplified and "enhanced" through a speaker system - so even what we like to think of as live performance is not necessarily uncolored sound.
You want "flat" response? Hire a competent a piano trio, sit them in a spacious living room with decent acoustics, and ask them to play. That's "flat." How you perceive what they play, however, will still be a function of the way your particular mechanical and neurological auditory sensory components are wired.