The 990 (and any surround processor) automatically detects the incoming signal format (Dolby Digital, DTS, or PCM) and behaves appropriately. Like Jed, I haven't used the surround mode buttons on my 990 in a long while since each input retains a separate memory of the last mode used for each of the different incoming formats. In my case, I have the video inputs (cable, DVD-R, DVD, and Oppo) set to use surround processing for each format and the CD input set up to stereo mode for PCM.