I think it really depends, but generally speaking the headpohone jacks don't get as much love (i.e. $$$) in the engineering budget as a standalone or DAC.
My Luxman integrateds for instance don't have a headphone amp. They use a resistor in series with the main amplifiers. So.....
Plus side is you are hearing 100% of the luxman goodness, bad side is the impedance is fairly high and probably wont' sound as good with low impedance headphones.