That pre-amp is really designed to feed into a further stage of amplification. The second amplifier would hardly load the output, allowing it to reach it's full level. Most headphones impose quite a heavy load on the amplifier driving them and that design may not be able to cope. As SunnySkyguy points out, the effect of overloading the output will low volume and probably high distortion.
The 'pre-' in it's name is telling you it goes before something, in this case the main power amplifier. Its designed to make the microphone more sensitive but still needs more 'punch' to drive a thirsty load like headphones or a loudspeaker.
Brian.