You have the creative console launcher installed right?
Entertainment mode has the options for Dolby and DTS and are both configured On (external decoder) or Off (built-in decoder).
I was thinking that you played with these settings already and was just taking a wild guess the SVM was...
Not sure if this method only adjusts volume by lowering the aux input but you can try it.
Load up Creative Console Launcher then click on Smart Volume Management and turn SVM off.
Otherwise you might need to check your receivers manual for some hidden setting.