OK, I've done some research, and in the course of concluding that I'm going to try out Sennheiser's HD580's, I realized what the difference must be with a headphone amp.

First of all, the sound will obviously be louder. I dont know about a reduced noise floor, in fact I think that with the additional electronics of the amp added to the signal path, the signal/noise ratio will be higher(not much probably so it doesnt really even matter I guess). I think the real difference is that the amp is capable of producing enough power to reproduce the low frequencies without trouble, in turn reducing all the other bad effects that come with overdriving an amp(harmonics, distortion).

If youre wondering, I'm going to try building a CMoy headphone amp (http://www.headwize.com/projects/sho...cmoy2_prj.htm), and see how it goes. I'm also going to try to start using lossless compression! Oh, I'm so excited