Is it possible to set the default pan type to something other than add channels? Of all the options listed, the only thing I use is constant power. Can't find anything in the preferences to let me change that default. Am I not looking hard enough?
Yes, this is easy.
Just pick an Audio Track that has desired settings (Panner, FX, Mon Modes etc)
Right-Click somewhere on the Audio Track header (in Track View, not Mixing Console), select "Set Default Track Properties...." - you get some choices!