need help with my Oppo

Yes. And use copper, not silver ICs/cables.

The purpose of the buffer is to present the driving component with a very high input impedance and the driven component with a low impedance drive. The appropriate application for the product is when the driving component uses an inexpensive op amp output stage, think DVD players and cheap CD players here, or with very long cable runs that are adversely loading the driving component. Your preamp probably has a more robust output stage than your OPPO, so using the buffer between preamp and OPPO makes sense and is free to try and will not hurt anything. Some also report a nice softness that the tube imparts in its distortion signature.

To take the edge off the Oppo go into the menu and do the following:
Speaker output to L&R (not stereo)
Upsampling to 96 (44 is to forward, flat, and harsh. You may like 192- I thought it was too spacious with a light bass)
Set the digital out to RAW (not PCM, even if you are not using an outboard DAC it affects the sound).
The sound will now be more smooth and involving.