The good news: I found my DJ mixer. I had indeed put it in its original 2005 shipping box, which is what a Smart Knuckles does.
The bad news: I think my shiny new MOTU M4 is borked. I cannot get sound out of channels 1-2 no matter what I do. The device is class compliant (does not require drivers) and it worked (or rather "worked") out of the box. I got Traktor to recognize it, though it showed up in the device list as "Audio Out." I assiged the two decks to outputs 1-2 and 3-4 respectively. The appropriate volume bars on the M4's display lit up when I play back a track on either deck. Channels 3-4 worked just fine. No amount of swapping cables and inputs on the DJ mixer could elicit any sound from 1-2.
I then installed the M4's Mac drivers (certified to work with Monterey) and enabled the extension (everything is disabled by defualt in newer versions of MacOS.) That caused the M4 to show up in the device list under its own name, but it didn't fix the lack of sound output on the first two channels. I've powerecycled everything (including the DJ mixer) but nothing has worked. I might try the M4 on a different machine, but I STRONGLY suspect my M4 is faulty
So, uh, no show next week I think.