Not a mac solution -- and possibly off-topic -- but if you're looking for *SUPERB* audio output, check out the Squeezebox from
http://www.slimdevices.com. Just came out with the third generation squeeze boxes and the sound (on my 2G) is fantastic. Burr-Brown DAC, 802.11g w/WPA and WPA2, etc.
Easy integration with iTunes, too. (Although, no, it won't play tracks purchased through iTunes because of the DRM -- but it'll play everything else -- OGG, FLAC, AAC, MP3, you name it.)