Gonk is correct, USB is a digital interface, it can only transport zeroes and ones, and the RR2150 must have a DAC in its USB port to translate the zeroes and ones into something it can send to the speakers.

That said, the RR2150 (and I would suspect the 990) is dependent on the computer, and the chosen media player to take the file, whether it is WMA, Flac, AAC, MP3 or whatever, and turn it into something that the RR2150 can understand...PCM perhaps?

Otherwise, the manuals would have to list the file formats the DAC could support.