Try this with your DAC connected to your Mac, and MC not running:
Apple Menu -> System Preferences->Sound
Under the output Tab make sure you have selected your DAC.
Under the Sound Effects TAB, choose internal speakers.
Quit System Preferences
Next go to your Applications folder:
Open the Utilities Folder
Launch Audio Midi Setup
For Built in Output ensure your DAC is selected and set the format to the highest sample rate supported by your DAC, which I believe should be 192000 Hz.
Close Audio Midi Setup
Lastly do this:
Launch MC
Go to Tools->Options->Audio
Under Audio Device ensure your DAC is selected
Then under Device Settings try enabling Integer Mode and Open Device with Exclusive Access if these are not already enabled.
Click OK.
And then one final thing:
Tools->Options->Audio
DSP & Output Format
Under sample rate just set all values to "No change"
Click OK.
Now see if you still have the issue. Also I just have to ask, so don't be offended, are you certain the 96Khz FLAC files are 96Khz/ If you highlight one of these tracks what Bitrate is shown?
Only other thing I can think of is trying a different USB cable, just to eliminate all possibilities, or try plugging your USB cable into a different UCB port on the iMac.
Barr