if you only have the 2 speakers then just leave your receiver on 2ch stereo. if you have a full speaker set up you can set the receiver to auto and not have to worry about it. make sure you go into the menu of the directv and turn on the dolby digital or it will only send out 2 channel audio.
dtv; menu>settings>audio>dolby digital
there are a lot of DSP options, i think that most people really don't use them. i know i could do without most of them.