I already know that mine does this same thing without testing as I have experienced this. The only way to adjust the Upsample or any other stereo mode is with the 2-channel offset. You should be able to measure a difference when using this to adjust the sub level in Upsample mode.

I figure Outlaw designed it like this due to the fact that most people seem to like more kick to movies, but then want to be able to back it off for music (I belong to this club).

I think we are on the same page now. Let me know if I can help any other way as I used to have a very similar sub setup.