Silly question - Why do you need a dedicated headphone amp?
Is it better than the standard outlet on my seperates amplifier?
Or is this just audiophilia?
Very generally speaking the headphone socket on an integrated amp will be ok, but won't have had a huge amount of thought or expense thrown at it. Some of the very high end amps have great headphone amps, but usually not so much.
A separate headphone amp can make quite a big difference, especially if you have a pair of difficult to drive headphones or some such as the one on the integrated amp will be designed generally for easy to drive headphones.
Where they can help a lot if you re really into your music is coupled with an iPod, iPhone, media player etc. The Apple stuff and most other recent smartphones have the ability to send a line-out signal from the dock port which you an run through a headphone amp and significantly improve the sound quality over the headphone socket. I use a little Fiio E7 with my iPhone and the change is quite appreciable - a much more open sound space and lots more detail. The headphone socket sounds really compressed if you go back to it now.