CDP breaks and I move to computer-based...

Have to agree with Audiofun here, try the Wireworld Supernova 6 glass toslink, from the CableCo or GCAudio.com. Toslink to mini-toslink connectors I think is what you would need.

I have also read that the Van Den Hul Opticoupler is very good:
http://eugenehifi.com/VanDenHul/htm/van_den_Hul_Digital_and_AV_Cables.htm

The other thing you want to be sure of is that you have coreaudio set to output 24 bit. Then let iTunes worry about getting the files to core audio in the right format. TRy a 24bit download while you are at it and prepared to be blown away.

what is core audio? If it is set to 16 bit, will it still play 24 bit files? Are 24 bit files available on iTunes, and how are these produced? I think CDs have 16 bit resolution, so any 24 bit coming off of them is upsampled. What software do you use to do this and is the sound quality better than a downloaded 24 bit file?
I run a PC, just FYI. I also read something somewhere about turning off "kmixer," whatever that is. Do you know anything about that?

Core Audio is part of the MAC OS. Kmixer is windows- two different animals.

In a MAC environment typically you would set core audio to 24 bit- set and forget and let itunes upsample everything which it does well. The other option is changing core audio based on what file is being played and matching them so they are native. This requires stopping itunes, changing the setting in core audio to match whatever file is being played and then restarting itunes. All of this is done to avoid having core audio upsample or downsample which it does very poorly. iTunes ironically does it well.

24 bit files are not available in the itunes store, but itunes can play them. If you download a 24 bit file from a place like hdtracks.com it will come in FLAC- you then convert to WAV, ALAC, AIFF, any lossless codec that iTunes can handle. Unfortuantely iTunes cant do anything with FLAC so it needs to be converted with another piece of software.