My guess is that if the Sherwood only supports PCM, the 990 will only support PCM. Still, it's possible they implemented it a bit differently.

Depending on how the hardware is architected, it's possible they could do this as a firmware update in the future. If the USB interface is tightly coupled to the DSP engine, it might be possible to improve the firmware for the USB audio device and have it pass DD/DTS into the DSP. If they used an off-the-shelf USB audio chip and glued it to the DSP engine, this will be more difficult. I don't see this as the likely implementation, though, since the USB interface can also be used as a means to update the firmware. Chances are, it's done as either a USB transceiver attached to their main processor or DSP, or as a generic USB microcontroller with some interface to their processor or DSP.

Enough geeky rambling for now.