I have an Apple Powerbook G4, which I've used to feed a Musical Fidelity Trivista DAC 21 through the optical out on an Apple Airport Express. The sound quality was better than the sound quality directly through the analog out on the Airport, but it never lived up to all the magical reviews I've read about the DAC 21. I never hooked the DAC 21 up to a CD transport to find out whether the limiting factor was the DAC or the Apple Airport Express optical out, but I'll bet it was the Airport. Recently, I purchased a Scott Nixon USB Tube DAC, and hooked it up to the USB connection on my Powerbook. I've been very pleased with the results, which are much better than my experience using the Airport/DAC 21. If your iMac is close enough to the stereo to use a USB DAC, I would recommend it. I'm using a 15' USB cable, and it works fine.
Hope you find this helpful.
Mark
Hope you find this helpful.
Mark