As I posted earlier, if you have an HDCD with Peak Extend, it's very to tell what the Oppo is doing, because the decoded stream will be significantly quieter (i.e., lower volume), since HDCD drops the volume by 6 dB compared to the undecoded stream.
On many a HDCD player I've heard ... you can actually tell which HDCD CD utilized peak extend - before playing: the player/dac will sound a tiny, but audible click (like a relay), but only with peak-extend enabled disks.
I noticed this moons ago, I returned one HDCD player to the store because of this click. They initially had no idea what I was talking about, but when I took it back and pointed it out, it became obvious to everyone. So, we tested every HDCD player in the store, same "problem." Of course it's not a problem, and perhaps I shouldn't have made such a fuss over it ... so I went home with the same unit.
The PM HDCD recorders were good even without specific HDCD features enabled. Some of the craft labels, such as RR/FIM, well, they sound so damn fine that it makes the entire digital vs analog debate seem moot. But many non-craft HDCD disks I own, like Atlantic Records for example, they don't tend to impress nearly as much. In fact, it's often cut based ... each individual track can sound different. Some offer PE, others not. On others, the HDCD light would turn on/off depending on the track. I even had one classical recording in which the light went off mid track/performance.
Plus not all HDCD players were created equal ... like nearly everything else in audio, HDCD was/is a moving target.