We have two Onkyo receives in my house that I have used the optical out on my AK100 with. The older one cannot play anything over 24/96, whereas the newer one plays everything. If you are running the ak100 optical out through a receiver/device that is a little bit older (Onkyo receiver that can't play is about 6-7 years old) it may not play.
care to share pics of your ak100 running through your onkyo?
I have an onkyo a/v receiver as well but unable to find an optical in port in the receiver. Had to use a RCA straight from the AK100 HO to my sunfire amp in the end.