I'll take a stab at this, just off the top of my head.
Dolby Digital EX can only be used with a Dolby Digital 5.1 source. The only 5.1 sources you'll see on digital cable are HD channels (and even then it's not guaranteed). The rest of the time, you will see a mix of Dolby 2.0 and PCM stereo. In those cases, it is using Dolby Pro Logic II (or PLIIx, which is the exact same thing in a 5.1 speaker setup) to produce a 5.1 source.
By the way, if you no longer have a rear surround speaker configured, Dolby EX won't be showing up anymore anyway. Some Dolby EX DVD's may try to engage it because of the way they're flagged, but that'd be the only case, and even then it won't actually do anything (in fact the 1070 may be able to disregard the flag when configured for 5.1 output).