anyone using a headphone amplifier

[Wags]

iPod output is analog, no? That means the sound quality is going to be limited by the DA converters that are in it. All you would be doing with a headphone amp then is boosting the signal. If you were going straight digital out of your computer in a lossless format and not degrading it with iTunes then you would hear a big improvement with something like this (probably cheaper options):

http://www.benchmarkmedia.com/dac1pre/