Short answer: Unfortunately it can't be done.

Long Answer:
You are not alone in this. For a long time i used a G4 Cube as my music server. I thought it would be perfect for the job, until i found out it doesn't have an actual headphone/speaker output.
i've gone through a bunch of USB solutions. The iMic, Creative Soundblaster MP3+ (Which would give me mic/line in, optical in/out, and line out,) Creative Soundblaster Live (It is the 7.1 channel version of the MP3+ and didn't work at all with mac,) a few other no name run of the mill off the shelf external USB dongles, and the firewave. Sorry if it seems like i'm just giving a good example of a horrible run on sentence but it comes down to this. Quite a while ago I found out about the Aggregate Device in Audio Midi Setup. Over at Cubeowner.com a few of us tried to get it to work, hearing that you could hook up several USB devices together and use them as one to get multi channel, or multi stereo output. For all we tried we could never get it to work. I spent about 2 days trying myself to get it to work. Even called Apple tech support about it http://www.cubeowner.com/forums/index.php?showtopic=10244&hl= and being told that Audio Midi setup is not an Apple product and to call the manufacturer for help. I did tell her off in that instance. I finally got transfered to the techgeeks in the Logic Pro department and they told me it couldn't be done except for use in Garage band or Logic Pro.

I wish what we have tried to do could be done, but it is just there beyond our use.