Got it, thanks for clarifying, BIslander. Finally got my DD 5.1 surround off two Blu-rays I tested, Super 8 and Passchendaele. Issue wasn't sampling or bitrates, and I was wrong about TrueHD not being backwards compatible. As you and JHAz pointed out, Blu-rays containing TrueHD will be processed fine by my Dolby Digital 5.1 system. I'm sol for any BD's that have only DTS audio soundtracks (yes I know I can downsample those), but for now, Daddy's happy again! Now to watch my first Blu-ray, with full audio, Passchendaele.
Thanks for all the help - gave me the drive to figure this out and not give up!