Congrats on that NAD cd player. The main sonic attributes you are hearing comes from the higher quality Burr Brown DAC in the NAD compared to the Oppo DAC. The Burr Browns are known for their linearity, very low signal to noise ratio, lower IMD and revealing low level detail.

I have several cd and dvd players and the best sounding one is my Rotel 1072 player which also has Burr Brown DACs.

John