Hendrik, firstly thanks very much for your reply. I'm a huge fan of your work and what you've done for the HTPC community, so thank you for that.
I guess I wasn't clear in my question - apologies.
Here's my understanding, please let me know where I've gone wrong:
Full RGB goes above white and below black. The pluge used on the HD basics disc has outer bars that are below black and a background that is at the required black level. The idea being that to get the correct 'brightness' setting you should dial down brightness to the point that the black and below black levels look the same, ie the black outer bars become invisible. So I agree with you that you shouldn't be able to see the below black bars ONCE properly calibrated.
When I perform this test using TMT6 with projector brightness set too high, I can easily see the outer below black bars and dial down the brightness until they disappear just as I do with my standalone blu-ray player.
With JRiver, it appears to clip the below black levels so I can't see the outer below black bars to get the brightness levels correct. Yet MadVR RGB is set to 0-255.
Shouldn't JRiver behave in the same way as TMT6 in its presentation of video material? As I said, it seems to be clipping the signal when other video players (correctly, such as my standalone BD player) don't.
Thanks for your thoughts.