i don't know much about macs, or the software available for them, but i do know a couple of audio things
1. the audio quality depends entirely on what bitrate you have saved all your music at-- if you ripped everything at 64bps, it is going to sound like junk no matter what you use. if you took care to rip everything at at least 192bps, i don't think that it should sound any worse than any comparable head unit.
2. this is where it can get as easy or as complex as you want-- you could either use the crossovers on the amps for a simple setup, or you could go full active, using audio software and a soundcard, and have the computer control what amp gets what signal... there are many options here, and none of them are less 'correct' then others...
3. after looking at the faq's, go by the mac section near the bottom of the forum-- there is stome stuff (front ends) that should fix that, but i still don't think it is a good idea to leave the house without a keyboard...