Hi Matt and thanks again. As noted in the OP, the hardware does support 176.4. I can play a 176.4 .flac file. Actually during that, the numbers "176k4" appear on my DAC front panel. So I am confused as to why the popup does not offer 176.4 as an option for the .dff file.
Edit. Now the thread might as well end. I just tried it again after closing and reopening MC and it plays at 176.4, so I don't know why it was doing that before - I didn't change anything. Anyway, it's working. Incidentally MC did crash a few times while trying this today, so maybe it's just a stability thing related to my computer.
Sorry for my .dff/.dsf dyslexia.