To understand the possibilities and limitations for low frequency reproduction, you're going to need to read the operator's manual for your pre/pro or AVR, as well as the manual for the OPPO. I have a BD player which outputs IEC 958, HDMI and 5.1 RCA. Since I have a pre/pro produced before HDMI , I use the IEC 958 connection from BD player to pre/pro, and manage bass from the preamps DSP to get low frequency to my sub. You too can use the digital output from OPPO to your amp to get low frequency to your sub just by setting the amps DSP to output low frequency to sub. On my unit I set to "stereo + subwoofer.
Now, if you are listening to stereo from your OPPO's balanced or RCA output, the full frequency spectrum, including low frequency will all be sent to your pre/pro or receiver's left and right inputs; and, your full range speakers will reproduce the the whole frequency spectrum including low frequencies. In this arrangement there is nothing going the subwoofer, unless you get an analog crossover that routes to sub and to left/right mains.