i only listen to mp3s. i know this isn't the best format for audiophiles but i want to generate at least decent sound from an external drive and a macbook connected to an amplifier. please break down the difference in sound quality when connected via the headphone jack, usb port, and firewire. i assume the firewire could deliver best results but not sure. i have read sound quality of add-on like these devices is only appreciable when the laptop's sound card is inferior, so what is the quality of the macbook's sound card? i want to make this work around as cheap as possible, extra points to anyone with a cheap solution.