What seems to be the case for me is that as long as the receiver CAN use a particular format that I have chosen in the input ocnfig menu, it will default to that. For example, my Xbox input, I have the default simply set to Dolby Digital. As long as theres a 5.1 DD signal present, this setting sticks. When the sound drops down to two chnanels, the receiver uses DD+DPL-2xC. When a DTS signal comes in, it switches to that.

Try turning auto-poll off, see if that helps.