I'm after a headunit for my car with high quality audio, that does aux-in - i.e. something with a decent DAC, etc.
Requirements: rear aux-in, plays mp3 cd's, EQ settings that don't mulch up the audio too much, high audio quality, not excessively expensive - I'm not after many features.

Primarily the source is going to be a carpc, and the output an amp (alpine gear). The carpc is currently plugged directly into the amp and sounds fantastic but I'm after a headunit for various reasons. I plugged the carpc through the aux-in on a kenwood unit, and it sounded terrible, hence this question!

Bonus points if anybody can recommend a headunit that takes optical in (5.1 channel?), and can verify it sounds awesome.

Are you sure you had matched the levels properly, and the "terrible" sound wasn't just from clipping?

You might have more luck with this question at diyaudio.com's car audio section.