Someone asked this question at AudioReview.com, he got an answer from a DD/Dts engineer. Here is the answer:
"First, it is not 6.1. It is 5.1+1. Marketing people use 6.1 to make it sound more impressive. Dts has the only 6.1 format available to consumers.
Now to the receiver. It is a terrible idea. There is no way to align the systems rear center channels without delay. There is no delay on the front channels of DPL, for which EX is based on. Unless your rear center speakers are exactly the same distance from the listening position as your fronts and sides, that is vital. If they are, delay is not critical.
The signal has to go through D/A conversion twice, and A/D once. This does audibly degrade the sound, and adds noise to the process. If the soundtrack was mastered at 20bit depth, then it is likely that the DPL D/A conversion would truncate it down to the typical 16-18 bit resolution found in in most DPL processing in receivers, pre Motorola 56004 chip
The steering can easily be overwhelmed in older receivers handling digitally discrete signals. This is were "offical" EX improves on the DPL process.
Do not get me wrong, a DPL receiver will extract a L+R signal. It just won't be as clear, or clean as the real EX process.
Terrence"
Hope this helps.