digital connection from computer to pre-amp

i want to deliver a top quality digital signal for uncompressed music, probably from the usb port of an applemac to my primare sound processor/pre-amp. this has optical and electrical digital inputs; also RS-232. i want to use the dac within the sound processor as this has several useful surround sound modes as well as 2 channel stereo. i plan to buy a second-hand applemac (i have my eye on a powerbook) to dedicate to the task. can anyone suggest what would be the best way to link the computer and the sound processor?