I know of no DVDs that would only output in stereo. It's part of the spec! I have a couple of music video DVDs that are recorded in Stereo only, but that's not what I'm getting at. I think the DVD player is set to stereo mode, not the DVD itself. There should be a setting in the DVD player setup menu to tell it to output "Mutlichannel" or "5.1" instead of stereo. I think on my Denon 2930ci there is an option to output either stereo or multichannel for HDMI sound output.
LFR1100 Actives,QS10HPx2,QS8x2,EP800,M3x4,M3x2 (Wood),M5HPx2 (Wood),AxiomAir,ADA1500-8,ADA1500-7
| 
