Turnstyle's post in your link appears to confirm what I was thinking. Here's the quote that he received from Outlaw back in November:
The USB input uses the Micronas chip, one of the very best available. The DAC for the USB is self contained on the Micronas chip. It offers:
-Single-chip, USB specification 2.0 compliant, stereo audio D/A converter
-Supports up to 24-bit playback
-Independent adaptive sample rates of 6.4 to 48kHz for USB playback
-THD better than -90dB and SNR of typ. 96dB for D/A converters
On other words, the USB input goes something like this: USB connector on rear panel, Micronas chip, then stereo analog into the pre-amp section. The Micronas chip
is the USB port as far as a computer is concerned, and it accepts PCM stereo over USB then converts it to stereo analog.