well, i don't have a 990,i have a 1050 and used to have my 'puter spdif connected,and it will decode but the best i can get(if i want it) is pro-logic.although after some playing,i go directly to my monarchy dip to cal sigma 2 converter,and this setup gives me a VERY clean sound,vs straight to the 1050.
you should have decent results,regardless.
hope this helps a little