Anthem AVM50 setup question

I purchased an AVM50 here in the classifieds about 6mos ago. I'm currently running only 2 channel with Paradigm Studio 100's. I have my Dish Network cable box hooked up via HDMI but the AVM50 automatically switches to Dolby 5.1 mode and it won't let me switch to 2 ch stereo mode, so I'm losing a lot of sound info with only having the 2 speakers hooked up. Can someone tell me how to get it to 2 ch mode?