I'm guessing it's a processing limitation. I know Emotiva made a change to their UMC design to add second dual-core processor so they could do full post-processing at 192/24, which apparently wasn't possible with the single processor. In most cases, systems that use dual processors (including the 972/997), the decoding of the HD audio is done on one chip, and then any post-processing is handled on the second chip. My guess is that the Trinnov processing is enough of a hog that there isn't enough processor horsepower left over for additional post-processing (i.e. PLIIx).

I think there may be a possible work around, which is to truncate the HD audio down to 96/24, which apparently many manufacturers are doing so they can apply post-processing on the lower-end chips.

Interestingly, if you've followed the Cary 11a fiasco, it appears that when the units originally shipped, it wasn't possible to apply post-processing to the HD codecs, but a recent firmware upgrade has made that possible. Not sure exactly what Cary did to allow it, but I suspect there's some sort of monkey-business going on in the background.