Not sure if there's something I'm missing, but can't find anything on the AK or other sellers' sites (e.g. Moon Audio, B&H) that states that the AK300 or AK380 support digital output via USB. All they talk about regarding data and the micro USB port is the ability to load data onto the DAP.
On the Moon Audio website it even says "The AK70 supports USB audio digital out, first among Astell&Kern devices."
Happy to be proven wrong!
Cheers
Please see the firmware changelog for AK380:
http://www.iriver.com/company/notice_view.asp?selectPart=&findWord=&sno=129&page=1&n=93&irTop=0