Its really not about the bus compression. Everyone uses bus compression. Wallace takes off about 4-6db. If you think about it thats not a secret to anything
What are tracks doing before they hit the bus? Thats the question. If Randys snare is square then its hit with a clipper before it hits the bus. You dont make squares with bus compression.
There is no one thing he does. His talent is making each and every track super HiFi. They have maximum spectral information so they are as present and as deep as the song will allow. Im not talking about the black album--he mixes different now.
He just hears better than everyone else. Its not technique. Most of us can mold a single track into its sublime full spectrum glory but he knows exactly what voicing each track has to have in order to be combined into one gigantic sublime mix
One thing him and wallace absolutely do that
Is a technique-- is they use some form of Mid/side processing on the guitars. I dont know what their formula is but you can hear it Linkin Park, Thrice, Nickleback and others. It some kind of pseudo guitar blend that widens(but does not collapse in mono) as well as enhances the guitar voicing. Its not the standard phasing tricks or the standard M/S process
But its a total package where every track is so well thought out in regards to its sonics that it sounds like every single frequency in the spectrum is being represented to its fullest. Just as Einstein had this extra virtual blackboard in his mind to do equations, those 2 have the ability to blend tones in their minds.
If they didnt it would take 10 years to mix a nickleback record using trial and error in order to achieve such a robust mix.
But fret not. Some of the greatest mixes in history are just simple stuff. A lot of wallace stuff is only good
because of the mix. If the songs were good they wouldnt even need to be mixed so awesome. Led Zep anyone?