A.) The size of the soundstage is determined by the width between the speakers. with stereo at a normal listening distance, you don't want more than 8 feet or so between the speakers. If you push them any wider apart, you have to sit further back, which reduces the perceived size of the soundstage. But with a center channel, you can double that, with 8 feet between each of the three speakers. 16 feet wide plus the added throw to the side of the outside speakers brings the size of the stage up to around 20 feet, which is still manageable for a large living room, but puts the soundstage into a human scale. It's the audio equivalent of the difference between watching a 40 inch TV or seeing a movie projected on a ten foot screen. B.) There are two philosophies for mixing multichannel music... Ping Pong and Immersive. Prog Rock like Pink Floyd has lots of overdubbing of details and fills that lend themselves to the ping pong approach. The rhythm section is anchored up front, but the solos and fills get thrown to individual speakers- left, right, rear left, rear right- bouncing around the listener like a ping pong ball. Immersive is quite different. Instead of using each channel as a discrete channel to place sound into, the mixer creates a sound field from all of the channels combined. The sound meshes into the phantom center of all 5 speakers, so it can be moved to any point in the room by potting it a bit in one direction or another. The two mixing philosophies can be blended too. There is an Elton John SACD where the piano and drums are fixed up front to the left and right. The vocals come through the center channel. At the bridge, a guitar solo is introduced, but it is mixed immersively. It starts up front in the center, and as the volume increases, it's potted toward the rears. This gives the impression that the guitar player is stepping forward in front of the rest of the band to take his solo. If you close your eyes you can place the player clearly about 6 feet in front of the rest of the band. There are classical multichannel mixes that exactly reproduce ambiences of concert halls, opera houses or cathedrals. The meshing of the five channels creates a three dimensional ambience with different reverberation and decay to each side of the listener. You not only hear the music, you hear the size and construction of the space it's inhabiting. The problem is that with immersive mixes, the devil is in the details. You have to have a really well suited room and very carefully calibrated speakers to get them to mesh properly. If they are poorly meshed, the phantom centers drop out and the sound starts to come from the corners of the room, not the middle. Most people who have multichannel systems use it primarily for movies and don't bother to go the extra mile to properly calibrate it for music. That requires precise EQ to assure timbre matching of all of the speakers and careful level balance to bridge the center without making any single channel overbearing. Not as easy as it sounds, but it is possible. C) Sgt Pepper is an album that completely lends itself to the most complicated multichannel mixing techniques. There could be sections that could have an open air ambience contrasted against parts with a very enclosed small ambience. When they pull apart all of the overdub tracks by going back and rebuilding them from the 4 track premix elements, they'll end up with close to a full 24 track master full of separate parts. That will give them a lot of flexibility to create any kind of multichannel effect they want. 5.1 creates a sound field that is a flat plane- left, right, back, front. Atmos adds speakers above to create a plane like that above the listener's head. This creates a cubic sound field. Using immersive mixing, this makes it possible to not only place sound in the left/right/front/back plane, but in a vertical dimension too. Sounds could be placed anywhere in the room at any height. Music could erupt out of the floor and fly into the air above like a volcano. It could fill the room from the bottom up like water filling a fish tank. You could have one part of the music moving across the floor from front to back while another part goes above your head from back to front. Think about how Strawberry Fields would sound if the music was handled like flowing water. "Picture yourself on a boat on a river" "No one I think is in my tree. I mean it must be high or low." There are a million possibilities for using height to indicate being underwater or in the sky or swirling in circles. Martin did a multichannel mix of ta pastiche of Beatles songs called the Love album. It's a real kaleidoscope of what multichannel music can do. Before it came out, people were scoffing it saying "What can be done with 40 or 50 year old recordings?" They didn't think that multichannel could add much. They were dead wrong. I think the same can be true of Atmos. It still hasn't been confirmed that the blu-ray will include the Atmos mix. It might just be for theatrical demonstrations. Like I said before, it requires a carefully designed multichannel installation to get the most out of these sorts of mixes. Not everyone with a 5.1 system is going to be able to pull it off. EDIT: One side comment... there was some discussion of how most home 5.1 systems are "adequate" to create coherent multichannel audio. This isn't true. Most people with 5.1 use the "home theater in a box" kits with tiny satellite speakers on stalks and a woofy black sub on the floor. It's almost impossible to get a sound field to mesh with a system like that because of the extremely limited frequency response of the mains and massive dropouts in-between them. What you end up with is sound coming from the corners of the room and nothing in the middle. In order to create a coherent sound field, you need full range speakers all around and they need to be EQed and level balanced so sound can be handed off from any of the other speakers without a dip in the middle or a timbre shift.