The 990 will not (as far as I've been able to tell) directly decode MP3 files. When a computer is connected to the 990, the computer needs to handle playback - there is no interface for directly accessing the hard drive. As a result, if you have a software player installed that will play back an MP3, a digital audio output (or the USB output) will be delivering an uncompressed PCM bitstream no different from a CD player's digital output.