Did/do both units feed the same preamp/amp? If yes, might be an issue with the Emotiva XDA-3.
Both models have similar output: 2.1 Vrms (OPPO) vs 2.0 Vrms (Emotiva), unless you were using balanced-out on the OPPO (output = 4.2 Vrms).
I looked out of curiosity, seems the Emotiva website and user manual for the XDA-3 lack specs on the headphone amp, which is odd.
Here’s some info on Vrms-matching between DAC-amp:
For the headphone amp volume, I’d read up on the model or contact Emotiva for more info.