Human beings are remarkably good at compensating for deviations in frequency response. You can very quickly acclimatise to deviations much greater than +-10dB.

Human beings are remarkably good at compensating for deviations in frequency response. You can very quickly acclimatise to deviations much greater than +-10dB. The main shortcoming of mid-priced headphones is transient response, which we are acutely sensitive to. Heavy, flexible and poorly-damped drivers in highly resonant enclosures might look reasonably accurate in the frequency domain, but they have absolutely dismal real-world performance. Audio engineers routinely choose transient response over frequency response, hence the popularity of Yamaha NS10 and Auratone 5C monitors in professional environments.