I suspected as much. To answer your questions.
Yes it is a Stereo DSD192 Dac, but you need to update the firmware to the latest beta version (1.7.5) otherwise it doesn't work.
Yes I can play play a redbook aiff file as 4 channels to the Mytek, well thats what audio path is currently telling me. It is saying "Convert from 2 channels to 4 channels" and I can hear music.
Paul