The Denon receiver you have does not support the decode of flac files so bitstreaming to it will not work.
I would suggest setting the output to directsound rather than WASAPI to test the output from MC28 as you say the Windows control panel is reporting 192khz 24bit capability. Using directsound will send the output via the Windows mixer.
If this gives good results then perhaps look at your WASAPI settings, it might be worth trying 'Disable event style'.
If you still have problems when using directsound I would suggest connecting your NUC using the same cable and input you use for the Oppo as that appears to be working OK, just to eliminate any cable/HW issues with the Denon.
Have you tried using the MC28 Tools>Advanced tools>Audio calibration to play test signals? (it can generate 5.1 output at sample rates up to 192khz) This would eliminate any issues with flac file decoding.
Terry thanks so much for your reply...By Disabling Event Style I'm able to get 192K stereo files to play perfectly YAY!!
5.1 96K files are also playing now, although not at 24 bit output - I'm still getting the 16-bit output message: not using enough bits to output the input directly.
Then I played the audio calibration signals and this matches what I'm getting, in that it plays 192K stereo fine, but it can't play 96K 5.1. When I try 96K 5.1 I get the same 'playback cannot be started' error that I reported in my first post.
So I converted a FLAC 96K 5.1 file to WAV to see what happens in MC, and I'm getting playback but with the same 16-bit output message. So it seems FLAC decoding is not causing the issue here. Does this help to understand what's going on?
FYI I can play 96K 5.1 FLAC files in Foobar using both WASAPI (EVENT) and WASAPI (PUSH) settings.
And thanks for pointing out that my AVR doesn't do FLAC encoding, that's great to know!