First and foremost, about the flat discussion, Yamaha officially provides the frequency response
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So their expected frequency response is known at the time of buying.
But, and this is important (as it seems that nobody in this thread seem to use them correct) this speaker is for near-field monitoring. Hence they are called, Near Field Monitors. This is the expected frequency response when using them correctly. If you misuse them for far-field listening, don't complain about them performing different.
And yes, of course, as you expect from the cheaper model, they don't measure as flat as the more expensive model
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But you calibrate your speakers anyway, so it doesn't matter that much. Almost every Sound Studio i know uses an calibration mic, calibrates them with their correct values they put into their DSP and done.
Please do not use any speaker for critical listening without calibration, even Neumann recommends two calibrate their razor flat KH 120 II because they can not know your room.