I am not an engineer or even remotely educated in the realms of audio tech, but I am gonna take a shot at this (a guess if you will). My observation with my particular setup has been that TrueHD and DTS HD tracks have much more active surrounds. The effects are also much more isolated and localized. Many standard DVDs I have watched, I would only get isolated effects here and there. (I purchased my first dvd player with dolby digital when dvd players hit $200 so I have watched quite a few in my day!) I remember really only hearing gunfire, rain, etc. here and there. HD audio blew me away when I first heard it on bluray. There is constant presence, more discrete sounds and atmosphere. Now, your receiver has to drive those speakers. More sound requires more power from your amps. I'm gonna guess that unless you are running a high end setup with separate power amps, the more speakers you drive, the less watts/channel you are putting out, requiring more volume to get the same perceived effect. Like I said, I am by no means a professional, just taking an educated guess. Someone with more technical know how could probably give you a better answer.