@cd318, 5.1 surround sound has an advantage of adding the center channel. Stereo was originally 3 channel and as you know two channel depends on a phantom center. Having a center channel in place makes the system a bit less dependent on a phantom center but that is a preference, not a necessity. Atmos adds another dimension of sound coming from above you. Think of what speakers on the left and right do, Speakers above and below give the engineer more possibilities to mix with. Objects are placed in the mixing software and are rendered as closely as possible to the mix with YOUR speaker setup. Here is a clip about object based sound: