You have two DAC's and maybe a third one in the preamplifier. You are either sending undecoded data from the Oppo or decoded and converted audio depending on the setup. Assuming analog connections between either the DAC or the Oppo, the first case would be connected oppo to DAC digital input to preamplifier analog input. The second case would be Oppo to analog preamplifier input. If you are using digital connections from the oppo to the preamplifier you are adding yet a third DAC to the mix. I'm sure the problems you encounter deal with setup and input selection.
While personally I wouldn't use an outboard DAC at all, if I had your equipment to set up, I would probably connect the Oppo's digital output to the receiver's digital input for movie watching. The music connection would be Oppo to DAC with digital and then DAC to preamplifier with an analog connection. That would take the Oppo completely out of the picture as an analog converter. Just follow each connection chain and do the proper component setup and input selection.