i am about to order a set of cantons (2xcd100/1xcd50/4xcd10/1xas22sc). this is the wall where the main sofa will be if what i have read is right, the surround right and left speakers should go high up tilting down on the side walls, and the surround back right and left should be at the same height on the rear wall behind the sofa again tilting down. will that acheive the optimum sound? also, are presence speakers worth having? my yamaha amp has outputs for them, but im not sure if they will be worth having in the system? cheers!