So in the ten years that between The Downward Spiral and the 5.1 Deluxe 10th anniversary edition of The Downward Spiral, I probably listened to the album more times than there are days during that span. Figured I knew it front to back, and knew every tiny little sound that could possibly be on that album. And then the surround version came out, and at first I thought, there must be new stuff he added in here, since he had all the multitracks kicking around. And while for Big Man with a Gun, I'm pretty sure they re-recorded parts of that track, the rest of the album - all the 'new' parts that I thought had been added in... were there on my old CD, all along.
The Fragile takes that and multiplies it by at least fivefold. Have you ever inverted the phase on Just Like You Imagined, to where you can hear Mike Garson punching the piano keys, and the lower frequency synth buzzing that is otherwise unnoticeable in the thick of the mix? Or
heard the flute in Starfuckers? The multitude of layers in TDTWWA - even if it's just tracks upon tracks of guitars and vocals, modulating in the stereo field.
There's stuff buried in The Fragile that you definitely hear better in high fidelity, that would be more audible in a surround mix.