Fixing the DAC to the headphone also allows for cheaper higher-quality audio. Cheap driver + fixed EQ from hardware characterisation = perfect response without tuning, within the minimum impulse response level of the amp&driver (i.e. any imperfect driver and/or amp response is inverted in software prior to the DAC). Plus you can add a switch to choose a desirable distortion (e.g. valve amp) if desired. This can be done with discrete components, but requires either a) Characterising the system with a calibrated mic and a non-zero amount of knowledge. b) Throwing money into a pit attempting to buy individual components, ALL of which need to exhibit a perfect response.