Wow, just wow. Guys, I have spoken to Roku technical support (as was made clear in my original post). There is no question that the latest Ultra provides the ability to convert from Dolby Digital Plus to Dolby Digital. Support have made that quite clear. In the settings menu (audio), there is a choice of options to pick from, PCM Stereo, Dolby D+, Dolby D, Dolby D+/DTS. and Dolby D/DTS. Technical support had me select and reselect the relevant options for some time last night. Nothing worked - and they have not the slightest idea why. But thanks for hijacking this thread with the suggestion that Roku puts in meaningless menu options and then offers technical support for the non-existent features. Really, thanks.
Please, if there are others who actually own a new Roku Ultra and are successfully sending 5.1 content to an older (Dolby D/DTS only) AV receiver, feel free to share what you can.