On your PS3:
On the PS3:-
Select Sound Settings
Select HDMI
Select AUTO
Check DD, DTS and all PCM boxes up to 7.1 are ticked
Save the settings
Go to the BD/DVD settings
Select Audio Output Format as PCM and then save the settings again.
As already mentioned, the amp won't be performing the decoding as the PS3 decodes the audio on board and output the decoded audio stream as LPCM. It cannot bitstream the HD audio to the amp for external decoding.
At the film's menu, select the HD audio soundtrack and once the film starts, you can press the display button on the PS3 remote to see what you're hearing.