I you can "set" you prefered mode...by pressing "save" button when you use it.
For exemple when a DD5.1 track comes to my SP2 it switches by itself to THX Ultra2 cinema in 7.1 mode, when it's a DTS 5.1... to DTS-ES mode in 7.1, when it's a 2 channel one... to simple stereo, aso !