Here is what it says in the book that came with the set.
Disc 11 Blu-ray: Atmos & 5.1: The Original Album & Bonus Tracks
BY STEVEN WILSON & RICHARD WHITAKER
The main challenge with creating a surround mix from a vintage album like Who's Next comes from the relatively economic fashion in which it was recoded, at least compared to today's computer based recording where there are virtually no limitations. Several tracks, including 'Won't Get Fooled Again' and some of the non-album singles, were recorded on 16-track, but most of Who's Next itself was recorded on 8-track tape using the technique of bouncing down (multiple parts recorded separately but then reduced to a single channel on the tape to free up channels for further overdubs). Sometimes the multi tracks have the drums recorded in stereo, but just as often on a single mono track. Then there might be a track of bass guitar, a couple of guitars, a lead vocal, one or two backing vocal tracks, and an extra channel with piano, synthesiser, brass, or percussion.
To make a 5.1 from these elements mainly involved keeping the core rhythm track in the front speakers, lead vocals in the centre speaker (but not exclusively), and using the rear speakers for things like backing vocals and the aforementioned extra elements. That sounded beautiful. But then we come to Atmos. My studio is based on a 7.1.4 Atmos setup so I have potentially 11 speakers to fill with sound, including four elevated speakers. How was I going to make that work?! Well, the answer lay in the further elements added in the mix - effects like reverberation on individual vocals and instruments. For example, the organ on 'Won't Get Fooled Again' is bathed in a halo of reverb, which needed to be added during the mix (it's not recorded on the tape, reverb is nearly always added during mixing). Recreating this meant I could move the organ and it's halo of reverb separately in the surround field, making for a really immersive swirl of sound. The same is true of almost all of the reverb and echo effects on the album - in recreating these I was able to control and position them separately to the source sound to make the instruments and vocals sound multi-dimensional in Atmos.
Sometimes we were lucky enough to also have an original session reel. This might have numerous run-throughs of a basic backing track the band played, before a take was selected and bounced down to a stereo pair on another reel for overdubbing. Having this reel meant that I could resynchronise it with the master reel in the computer, which in turn meant that not only were we going back an audio generation in terms of the drums, bass and rhythm guitars, but also that I had separate control over the sound and positioning of these instruments.
For me the economy inherent in the recording process of Who's Next actually makes for a really engaging surround experience. You can hear the space between all the instruments, every gesture in the music. Even the sound of the room they were recording in. Both compositionally and performance-wise nothing on Who's Next is superfluous or wasted, everything counts. It's almost a cliche with surround sound to say that it's like you're in the studio with the band, but that really is the way it feels on these iconic recordings. You can almost reach out and touch the music.
The phrase 'classic album' seems almost inadequate in the case of Who's Next. What an absolute honour then, to be able to deconstruct and reconstruct one of the most legendary and influential recordings in the whole rock canon - as well as 14 non-album tracks from the same era - and present it in this new form, which I hope is simultaneously faithful to the original while presenting it in a fresh way for people like myself that have been in thrall to it for most of our adult lives.
Steven Wilson, August 2022
Note: Unfortunately the original multitrack tape for 'Bargain' could not be located, so this track is present in upmixed form in surround. For this I used Penteo, sophisticated software which takes an original stereo master and processes it to create a pseudo surround mix. While this is obviously not the same as being able to create a properly immersive mix by positioning individual elements from a multitrack tape, it is the best possible solution given the circumstances, and at least preserves the flow of the ablum when listened to in 5.1 or Atmos.