You have to go into the settings on the PS3 for this. Go to the sound settings menu and select audio output settings, you then get a choice of connection...i presume you are using hdmi so select that and then select manual. You then get a list of all the sound settings the PS3 can send to your receiver, if you want the Dolby Digital and DTS soundtracks from games and movies as they are on the disc then deselect every Linear PCM option in 5.1 and 7.1 leaving the linear PCM 2 Ch options selected and everything else selected.
You will then get DTS HD Master audio and True HD tracks played as they are on the disc and games will play in the audio encoded on the disc without the PS3 processing the audio first which is why your amp displays multichannel audio, it's an easy fix and won't take long.
Hope that helps.