this is the initial comment by the developer:
"Greetings,
My name is Jonathon Kennemer and I am the designer of the Simpl A1 Headphone Amp. To answer your question about the benefit of a headphone amp requires a somewhat technical explanation. Basically, the A1 amplifier's input impedence is very high, so the iPod's amplifier doesnt have to work hard at all to send the audio signal to the amp, which means far lower distortion. Second, the A1 amplifier's very low output impedence means that headphones will get the full current required to drive them properly. Our amp also has a playthrough feature so that it plays at normal volume when connected to the amp, with the power off, then engages the amp when you power it on, so you can very easily hear the difference with and without the amp. Furthermore, the iPod rolls off its low frequency response at 50 Hz when connected to 32 ohm headphones. The A1's high input impedence moves the low frequency rolloff to below 20 Hz, so you get much fuller bass response."
further:
"In response to your question about the difference between using the line out port as opposed to the headphone port, there are a couple reasons why we are using the headphone port instead of the line out. First, is simplicity. By using the headphone port, we retain the use of the volume control on the iPod. The iPod volume control doesnt work through the line-out port, so a volume control would have to be part of the amp. Second, the low frequency roll off through the line-out port is below 20 Hz, which is one of the reasons it sounds better than the headphone port. Our amplifier specifically addresses this issue and effectively allows the headphone port to operate full range , from below 20 Hz to over 20 kHz. We have tested our circuit both through the line-out (with an added volume control), and through the headphone port, and can not hear a difference. Our design philosophy is to keep things as simpl as possible, hence the name Simpl Acoustics. Thanks for the great question."