"Flat" by measurement is not the same as "flat" by ear since human hearing is not linear.

If you want to EQ by ear and want a "flat" sound, basically what you want is equal loudness from 10Hz to 20KHz. IEMs are more capable of this than headphones in regards to distortion.

If you want a "flat" sound by measurement (dB SPL) (but doing it by ear) then there should be a gradual roll-off from 1KHz down to 10Hz, a -3dB dip around 1.5KHz, a rise to +3dB at 3.2KHz, then a gradual roll-off up to 20KHz.

I think a happy medium is somewhere in between. Most people like a bass boost and treble is subjective too. Listening level matters. Having a smooth response is key so no stabbing peaks or troughs.