This question (in one form or another) seems to come up a lot. The general concensus here is that a good headphone amplifier will be an improvement over the headphone jack of a receiver. How much of an improvement will depend upon a number of variables--such as, but not limited to--the quality of the receiver, the quality of the headphone amp, the headphones used, and the person listening to the equipment. Because there are so many models of receivers and so many models of headphone amplifiers, it is very unlikely that any individual will have compared a specific receiver with a specific headphone amplfier. Therefore, it becomes necessary to deal in generalities. It is possible that a receiver will have an outstanding headphone jack (though I personally believe this is less likely for AV receivers).
There are a number of reasons why this is so. I believe that it has do mostly with money. The majority of people who purchase an AV receiver are not making their decision based upon the quality of the sound out of the headphone jack--if they listen to it at all prior to their purchase. However, some purchasers want to have the feature and it may be useful in terms of marketing a unit, so it is included on some receivers. (I would say it is more of an afterthought than a primary design goal.) In contrast, a headphone amplifier's primary purpose is to amplify headphones, as opposed to speakers. Therefore, it is designed and optimized for the electrical characteristics of headphones. Headphones and loudspeakers are not the same things. So, an amplifier optimized for headphones does a better job of driving headphones, and an amplfier designed for speakers does a better job of driving speakers.
Is it possible to build a good headphone amplfier into a receiver? Certainly!
Why, then, isn't it done? Because the great majority of persons who buy an AV receiver don't care. Because the customers don't care, the companies who make receivers don't put much time, effort, or money into the headphone jack. However, they do include it on certain models for the customers who want to have this feature.
Is the headphone jack on your receiver any good? I don't have the slightest idea. It may very well be "good enough", especially if you don't use it a lot or don't listen critically. But, if the quality of the sound is important to you, if you listen through headphones for extended periods of time, or if you are a critical listener, then a dedicated headphone amplifier will probably improve the sound.
