first, he has alc892 onboard not 1150. while true that high end boards with good onboard audio should handle his headphones fine it seems like he has rather cheap onboard sound which may not have a very good amplifier.
second, an amplifier is not for improving the sound but increasing it in power so that it may move harder to power drivers. while a good quality amplifier can certainly sound better than a lower quality one thats a different thing entirely.
the zero is rated at 150ohm impedence yet has fairly high sensitivity. if they were low sensitivity then i'd say an amp would be required although since they are high sensitivity its a harder call. given the low quality soundcard onboard and the fact that the zeros are fairly decent...