So to summarize what you've said:
1. Both the M2Tech HiFace DAC and the KingRex UD384 "WILL" play my 24/192 files directly from the USB out on my MBP without any additional equipment needed because they are both USB Audio Class Two compliant.
2. The Asus Xonar Essence One is NOT USB Audio Class Two compliant, and therefore would not serve much purpose to me if my goal is to play the 24/192 files as simply as possible.
3. I could not use either one (M2Tech or the KingRex) in my high end room system because the Sony Blu-ray player only has an optical (toslink) out and neither one of the two DAC's mentioned has an optical "in".
4. For computer use, they are both fine and I could send the "computer-based" hi-rez files to my home system via each of their respective output formats--mini to RCA for the M2Tech and "RCA to RCA" for the KingRex--or, to my tube headphone amp via the same connections.
This might be a stretch, but do you know either one of these units and do you know how they actually sound? I am going to assume from the reviews (no personal experience) that the KingRex sounds better. It seems to be designed for an audiophile audience. Plus it has the USB to S/DIF converter to send hi-rez files to other DACs in case I want to get a separate DAC for my home system.
Thanks for teaching me about this complicated, highly technical world of digital audio.