On the 990, it'd be the 990's DAC because the signal goes through the DSP section (downstream of the USB module) before being converted from digital to analog. As for the computer's PCM bitstream, it can probably do either. Playing a CD straight from the tray would presumably be 44.1kHz unless there was some processing or manipulation taking place, but I'd expect that bitstreams derived from material like MP3's or Dolby Digital sources would be 48kHz. The 990 can handle either, though, so it shouldn't be an issue if sources like that default to 48.