Brap, congrats. The speakers look beautiful and it's fun to hear your excitement.
To turn off the Oppo bass management (and the 40Hz rolloff) just change the speaker size from small to large. I would also change the subwoofer setting to off, and set the other surround speakers to off.
From the manual:
1. Size: The following speaker size parameters control bass filtering for the speakers.
x Large – All audio content is passed to the appropriate speaker.
x Small – All audio content, except for bass frequencies, is passed to the appropriate speaker.
The purpose is to reduce possible distortion. If a subwoofer is available, bass frequencies are redirected
to the subwoofer, otherwise, they are discarded. The bass frequency upper limit is set
by the Crossover Frequency selection in the Setup Menu (see page 73 for details).
x Off – Speaker for the corresponding position does not exist. Audio for the missing speaker is
redirected to other speakers or discarded if redirection is not possible.
x On (for Subwoofer only) – Specifies that a subwoofer is available.