For the most part there is no point or need. A DAC with flat response will accurately output any EQ changes to the signal. A headphone amp should accurately portray those changes as well. Now excessive EQ could overdrive the amp and cause clipping or high distortion. It also could do that for the headphone itself. Within reason, any EQ will get correctly transmitted out to the headphone response within the limits of the headphone.
Only if it was not meant acoustical output measured with microphone/ear coupler/dummy head after the EQ has been applied - and how it compares to unequalized output.
DAC alone should not have any audible deviation from flat.