Thank you for that, Kadiddle. It may be that my unit does not operate as it should, which was something I had hoped to discover with another owner testing. Maybe my best bet is to buy another one to test it and see if it performs as my current one does, since it is still a little hard to fathom a repeatable, consistant malfunction of this type where nothing else seems to be going awry.
My best test for you might be to turn on only the multichannel amp while a low bass Dolby Digital track loops -- all speakers set to full range except sub is NOT USED. Listen to the passage, then add the sub back in by menu and with the test tones, but never turn on the sub amp, just listen to the passage again through the multi speaker set. It is not to say there is NO bass -- there may even be quite some amount of it, just not like when sub NOT USED is selected.
I do have dual ten inch drivers in my mains and center, so that might be a factor, but I am thinking if I do feel tempted to send this one in, it will return exactly the same way, in which case I will have to resubmit, etc. I have been down that path before more than once -- not fun.
Maybe the only way to be sure is just buy another one -- that is not ideal either, of course.