Hitsofmisses hi.
Your Oppo has a digital domain volume control, are you using it's 5 channels direct into your Parasound Halo A51, and the sub output into your sub?
As this is the way I would use it, so long as the digital volume control is at or above 75% of full output, if lower you may run the risk of bit stripping, which is not harmful just robs a little definition the further down you go from 75% of full.
From What/hifi measurements
"Please ensure you enter the setup menu and set volume to “Variable”, then lower the volume control to an initial setting of 45-50 before powering on your amplifier or active speakers after factory reset. Volume setting is retained in memory when the player is put in standby. So volume at power on remains the same as volume at power off.
Output impedance 7.1 board L/R front channels 100Ω
Output impedance dedicated stereo board L/R channels 100Ω
Output impedance 7.1 board all channels except L/R front 200Ω
Recommended amplifier input impedance is 47KΩ (Industry standard)
Peak output level: 2.1 Vrms (RCA / Phono)"
All these output impedances and 2.1output volts are a perfect impedance match for the inputs of your Parasound Halo A51 and whatever sub you use.
Cheers George