Ok you are best to keep you Mac mini running via toslink to your HK 990 and set your midi set up to 96/24 for HD Music Tracks. Don’t run them in iTunes but do like me and use VLC. You will get the best results possible. Utilizing the HK Built in Dac.
I currently run mine thought a musical fidelity xdac v3. Run the Straight Line in with no processors in this case.
Cheers,
Mitch