No rear speakers in Pro-Logic mode

[baffledboy]

Ok, I'll explain this problem as best I can. I have an external amplifier (Sony 3200) which is fed via digital co-ax output from my HTPC. If I play something with either Dolby Digital or DTS sound, everything is just fine. No problems at all. However, if I play something with ANALOGUE sound (a TV programme or audio) the amp should output the analogue matrix as pro-logic. However, I get no rear-speaker action at all in this mode. The amp doesn't even ackowledge that anything is being fed to the rears. I'm assuming there's some sort of down-mixing going on before the audio reaches the amp, but I don't know where to look. Anyone got any suggestions?

Cheers
S


TV-Server Version: MP1 RC3
MediaPortal Version: MP1 RC3
MediaPortal Skin: Blue3
Windows Version: XP Pro SP3
CPU Type: Pentium Core 2 E2140
HDD: WD 180GB SATA
Memory: 512mb ddr
Motherboard: Asus P5LD2-X/1333
Video Card: Nvidia Geforce 8500gt
Video Card Driver:
Sound Card: ONBOARD
Sound Card AC3: Coaxial out
Sound Card Driver:
1. TV Card: Hauppauge WinTV Nova -PCI
1. TV Card Type: DBV-T
1. TV Card Driver:
2. TV Card: Hauppauge WinTV Nova PCI
2. TV Card Type: DBV-T
2. TV Card Driver:
3. TV Card:
3. TV Card Type:
3. TV Card Driver:
4. TV Card:
4. TV Card Type:
4. TV Card Driver:
MPEG2 Video Codec: Nvidia
MPEG2 Audio Codec: Nvidia
h.264 Video Codec:
Satelite/CableTV Provider:
HTPC Case: Ahanix D.Vine 5
Cooling:
Power Supply:
Remote: MCE
TV: Samsung LCD 32"
TV - HTPC Connection: DVI-HDMI