IIRC DPL II with the CES mode upconverts to 6.1 distinct channels with the 2 back channels being in mono, whereas I think DPL IIx upconverts to either 6.1 or 7.1 distinct channels. Incidentlaly, the twice.com link says
Quote:

Upconversion to 7.1 has advantages over 6.1 upconversion, some suppliers said. Systems with a single back-channel speaker fall prey to combing effects that reinforce certain frequencies, and it can be hard to distinguish if the sound is coming from the front-center or rear-center channel, they noted. For that reason, some receivers with EX 6.1-channel decoders feature seven full-range amplifier channels to support two back-surround speakers, each delivering the same mono signal.