I am happy to report that McIntosh tech support somehow guided my dealer through a successful install of the USB firmware. I wasn't able to speak to the programmer, so not sure what they did to get it to work?
Am listening to Hi Rez files now, and it is sounding very good. The C48 DAC has a slightly smoother, more liquid sound than my W4S DAC-2, which I find easier and more relaxing to listen for long periods.
Just noticed one small glitch so far. Sometimes when changing between files of different resolution, the preamp will stall, and needs to be power cycled to get it working again. This may be related to Bit Perfect in the way it queues files? If I remember to always pause a playing file first before switching between files, then everything seems to work perfectly.