Connecting iMac to AVR

Wondering if anyone can help with a problem that i have.

I have my iMac connected to my Audio/Visual receiver via hdmi (mini display port adapter)

I can get all 5.1 channels when watching films and even get DTS and Dolby surround sound via bitstream. the problem i have is when playing a film in stereo or a song via iTunes the amp is still receiving a 6 channel signal i.e. front left and front right.

The iMac doesn't seem to switch modes and send a stereo signal. as a result i am unable to apply any "surround modes" when watching a film that only has stereo audio track or use 5 channel stereo mode when listening to music.

I can can get the iMac to send a stereo signal using the "audio MIDI setup" program in the utilities folder and choosing 2 channel there and thus can use my surround sound modes - but then my problem is reversed i.e. all films with multi channel audio are received as stereo.

Every other device/appliance i have will auto change the output depending on what source is being played, PS3/Xbox etc

Any help greatly appreciated

Well I found out how to fix this so posting my findings incase anyone else comes across this issue. My issue seemed to be related to 2 problems.

found a post on apple discussions in which someone stated:

"To pass Dolby or DTS to your receiver or TV, you must
go to Audio Midi Setup and select your HDMI output and
set it to 24bit, stereo. This will allow pass through of
these signals to the receiver/TV.

This is because, Dolby and DTS are streams encoded
into a stereo digital stream. There is a flag in this stream
to tell the the receiveing device to decode it."

Once that was done I then needed to change a setting in XBMC (Media centre app) where i play i films from. I changed my output to Optical (which converts the AAC track to AC3) and kept my output "device" as HDMI.

I guess when i previously changed to 2 channel in mac OSX's midi device, XBMC was trying to bitstream AAC to my amp which cannot decode it. The option i changed is not really intuitive and should be labelled AAC to AC3 or some such not "optical"

hope this helps someone