Thanks for the suggestions.
So, a little experimentation goes a long way...
1. I tried the A5.5 USB input straight from a Mac USB output (CD file copied as AIFF file to Mac) -- not a disaster, but not great, sounded very compressed compared to CD. Not acceptable, and not the solution.
2. I tried the digital optical output from a Mac into the Toslink input of my existing MF X-Dac v3; again comparing CD to same files copied as AIFF to Mac. *Amazing results* !! Output is almost indistinguishable using Mac Toslink input to DAC compared to CD coax input to same DAC (output from DAC, regardless of source, goes to a MF X10 vs3 and then to MF A 5.5 integrated) with same source file on CD or Mac.
So, this is clearly the way to go - use the existing DAC with toslink output from Mac. No need to buy a new dac or halide bridge, just a dedicated mac Mini, and reconfigure some existing cables.
Happy days!