Balanced Output to Unbalanced Input
The short answer is no, there is no benefit to connecting balanced XLR outputs to unbalanced RCA inputs. Balanced connections are used to reduce the effect of noise induced by stray electromagnetic fields, especially in long cable runs. They include two signal conductors and a ground conductor, and the two signal conductors carry the same audio signal at opposite polarities as depicted above. Any noise picked up along the cable will presumably be at the same polarity in both conductors and thus rejected by the differential amplifier at a balanced input. Since there is no differential amplifier at an unbalanced input, any induced noise will not be rejected, and it will be amplified along with the intended signal.
Not only that, it's possible that connecting a balanced output to an unbalanced input might be detrimental to the signal, depending on the design of the balanced output stage. In most cases, the negative output of a balanced output can be connected to ground, but in some cases, it should be left disconnected. Assuming the Oppo player and Marantz AVR will be in close proximity to each other, I recommend using short unbalanced cables with RCA connectors on both ends in your case.
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