I have nothing for your particular situation, but I found that HDCDs sound amazingly great even on players that don't decode them. Often better than SACDs. Seems odd that it would be any different on the Oppo.
For OPPO owners. HDCD function
For many years I put the HDCD setting to ON in the settings function, with the confidence that Reference Recordings which use that coding would sound as they should.
I always wondered why those encoded CDs never sounded very good. Indeed they sounded flat and lifeless and at a very low volume.
Lo and behold, I turned OFF the setting today and the same recordings came to life with much higher volume and greater detail.
Is it my machine or a design flaw from OPPO?