There’s really no such thing as a ruler-flat loudspeaker.
Loudspeakers are such inaccurate mechanical devices that if one were to attempt anything other than a relatively flat frequency response, enormous levels of compensation would be required. And, even then, those compensating changes in amplitude would only work in a very small area.
Unlike an amplifier whose input vs. output is fairly straightforward, speakers present an enormous challenge—one that depends on so many outside variables (box, baffle, room size) as to make it a non-starter.
The price of flat, as it pertains to loudspeakers, is equivalent to the price of peace.
Not to despair. Fortunately, our ear/brain mechanisms are powerful enough to adjust so that when we listen, it sounds “flat”.
And at the proverbial end of the day, if it sounds flat then it is flat.