Ive just istall a mac mini and i am using a 5.1 USB sound card to send audio to amps, they use a headphone jack output, so i used a headphone jack to rca audio lead to connect to amps. It was pretty cheap at £29 from Maplins. As it is a full surround sound card you could have full dolby, but i am only using the Sub output and the headphone output for stereo.
Initiallly tried the sound through my old headunit, but it had a lot of interferance noise from hardrive and powersteering.
I'm quite happy with the sound quality now, im quite interested in using optical out but dont know if there is an car amp that has it.
Hope this helps.