Hi,
Sorry for late reply, I have tried the NA7004 Headphone out and also compared it with my dedicated headphone amp and Denon PMA1510 (inte. Amp) headphone out. My opinion is NA7004 has pretty decent & strong headphone out with clear vocal which I have not expected, My dedicated headphone amp (Audio-GD Fun) has just little better overall resolution and defined bass. Denon PMA1510 strangely has detailed response with it's headphone out which I never tried before.
Only one thing I feeling little disappointed about NA7004, it is their cheap volume control knob/pot of headphone amp which was producing some static noise during tuning up/down, was little irritating. May be it was due to remained as non-operational for several months.
Overall I think NA7004 quite strong, worthy & clean as a headphone amp and not really that far from the performance of a dedicated headphone amp.
The headphone I used for the audition was GRADO SR-80.