Originally Posted by d marc0
How ironic... I found that post by ClieOS and it was in response to your question!
Maybe we should ask @ClieOS maybe he has the technical knowledge to explain why...
Because of capacitor on the output, to put it short.
When you have a capacitor followed by a resistor (the headphone load), it forms a high pass filter electronically, so some low frequency signal will get filtered out. How much frequency is filtered out depends on both the capacitance as well as the resistance. If the resistance is fixed, then higher capacitance will lower the corner frequency (the -3dB point of the high pass filter). Vise verse, if the capacitance is fixed, the higher the resistance the lower the corner freq will be. Since most of us won't want to change the caps inside C3 (I tried actually, but there isn't enough space), the more sensible / easier thing to do is to increase the resistance. But simply increase the headphone impedance / resistance to fix bass roll-off is problematic in itself, as C3 isn't a very powerful player to begin with, not to mention the high possibility of coloration on the headphone. So here is where an amp is helpful, since most portable amp has input impedance over 5kohm, which is way bigger than any headphone you will ever find, it will push the corner frequency very low and solve the bass roll-off issue.