One suggestion from one source, totally anecdotal, non-verifiable, and as you state "not double blinded AB" does not constitute some known data.
We can determine:
The specs of the Oppo are comparable if not a bit better than the XSP-1 and the noise floor of the Sabre DAC @ -135dB is not bettered by the XSP-1.
There is no known problem or quality issue with the digital volume control on the Oppo-105.
Other devices connected between the Oppo and the power amp are most certain to color the sound in some way. In many cases this change, or reduction in transparency, will be pleasing to the listener, especially the one whose funds acquired said devices.