Can't seem to figure this out....I was on Dobly's website...it lists DD EX as adding a additional speaker, effectively providing for a 6.1 environment. However, I have several DVD's listing specifically DD EX 5.1 Surround Sound. What is the difference? The DD EX 5.1 (Return of the King, for example) does not engage the surround back speaker. So why is DD EX 5.1 any different from DD 5.1? Thanks for your help.