There are several buttons on the 990's remote (Dolby, DTS, and Stereo) that are used to scroll through available modes. Which modes are offered depends on the signal, so not everything is going to be available all the time. (5 Stereo / 7 Stereo, for example, only works with stereo inputs that are either analog or PCM, not with any Dolby Digital or DTS signals.)

You can look at my MX-700 file for an idea of how I've made them accessible (see "Remote Files" in my sig for my an assortment of 990 files and my MX-700 review for my complete setup file), but basically I have the buttons on page 2 of just about every device I have.