Surround Format Question

My system consists of a 7.1 setup using the 7.1 analog outputs on my Denon BDCI3800 Blu Player. My question is, will I only be able use the two surround back channels when the disc is mastered in 7.1. I was watching "We Were Soldiers" the other night, and it is written in DTS 6.1. I assumed that back speakers would playback in some form of phantom mode or simply in mono, but there was nothing coming from them. So am I stuck with exclusively 7.1 mastered tracks when it comes to getting any use out of the two surround back channels. Anyone have any insight??