Well, it looks like the whole DTS-MA 7.1 to 5.1 downmix debacle is the result of a misinterpretation of the AIX test; another poster in the Oppo thread posted a response from an AIX employee which stated the following:
Originally Posted by Brent555
Hi Folks, I'm Brent Curtis, I work for AIX, and we made the Blu-ray disc that came with the Oppo. Bob P. is completely correct. The DTS-MA downmix was set to the lowest possible setting to prevent users from playing a 5.1 stream when they had chosen a 7.1 test. We wanted to set it to silence. Regrettably, we should have mentioned this somewhere on the disc.
If you have the settings in your Oppo and AVR set correctly and really have 7.1, then the DTS-MA test will play at the normal volume. This test should only be used if you have a 7.1 system. While not DTS-MA, there are other 5.1 tests that can be used. For those people who are having troubles with the settings, we noticed some interaction between using the analog outputs and having the Secondary Audio turned on. Please contact Oppo directly for clarification, but first try turning off the Secondary Audio.
We're glad people are enjoying the disc and am sorry for any confusion this has caused, but in a way, the trick is doing its job in illuminating when the 7.1 is not really playing.
Anyway, based on the above post it looks like there is nothing wront with the Denon, Yamaha, or Onkyo/Integra receivers.