But previous announcements indicated that they plan to include PLIIz, which means the "rear surround" outputs are likely going to be able to be re-assigned as height channels. In that scenario, they could also be assigned to "DSX height" or "DSX wide" instead.

That being said, I don't know how likely it is that we'll see DSX appear on the Model 978, especially since they've made no mention of DSX. XT32 was a very late addition, and since the processor is focusing mainly on 7.1 they may have decided not to bother with DSX when they already had PLIIz.