Originally Posted by nymartyk
Depends on the preamp and the quality of the headphone output and the headphones you're using.
Very true. The requirements are that the headphone amp put out enough voltage so that you can listen at a comfortable level which can be problem with some phones that have high impedance and low sensitivity, that it not have audible hum or noise which can be a problem with some very sensitive headphones that are out there, and that it provide a low source impedance, which is particularly important with low impedance headphones.
Note that not all of the above requirements relate to every set of headphones.
Equipment that has dedicated active circuitry for driving the headphones has a better chance of hitting a home run and meeting every requirement that I gave above. But, just because a headphone jack has dedicated active circuitry does not mean that it does everything right.
I use a separate headphone amp (Bottlehead Crack) and it sounds 100x better than the headphone out on my Yamaha receiver. It's a better impedance match to my sennheiser headphones.
Thing is, you don't have to spend hundreds of dollars to get a headphone amp that is technically sound. In some cases the equipment at hand, such as a more recent iPod or a Sansa Clip digital player comes with a well-driven headphone jack.
The obvious price performer out there is the Fiio E3 which runs under $20. It is optimized for use with computers or portable players so it requires USB power and has a built in battery. You might have to spend an extra $15 or less for a USB power supply (such as is used with some cellphones) to integrate it with your home equipment. It also has a 3.6 mm input jack, so an adapter might be needed with the 1/4" headphone jacks on some home equipment.
Check out head-fi.org for more info.
Unfortunately many of us find that there is lots of pseudo-science, sales hype and disinformation there.