It does not support AAC. The DACs in the main pod/sub are 16bit/48KHz, but pretty nice quality. You can send 24bit/96KHz and 24/192 streams, but they are automatically downsampled at the DACs.
I've gotten to know my Z-680s pretty well. Best money I've ever spent on computer-centric A/V gear.
It handles AC3 and DTS over digital links, but not the WMA-Pro 5.1 nonsense, and not AAC.
It can decode MP3 sent over the digital link, but it's not designed to do it, and there's not much point.
Behemoth - Plex server
Galactica - FreeNAS
Pegasus - R710 ESX host
fsociety - MBP 15
Rocinante - E5530 testing machine
Saratoga, Ticonderoga, BunkerHill - the Hydra desktop PC-thing.