subwoofer buzz - how to eliminate it

It is placed on a wooden floor on rubber footers.

So why is it 'vibrating like it wants to fall apart' straight out of the box.

It's obviously faulty.  Take it back.

These get great reviews and are very reasonably priced. I'm thinking of getting some.

https://www.svsound.com/products/soundpath-subwoofer-isolation-system

I have the svs ones on my living room sub. Without it pictures on the wall would rattle with it no rattle.

I see this as a design failure. In my own design from 20 years ago and  which is downward firing, I have NO BUZZ or vibration. I KNOEW that with Bass frequencies that the BOX must be stable and weigh enough to prevent vibration @ frequency.

Of course my BOX (case) is 1/2 in PVC and weights in at about 80 lbs.

You can easily place your favorite family photo on top and never worry about it sliding off the top.

Check out Primacoustic Recoil Platforms. I have them under each of my three subs.They work best flat against the bottom in between the feet.Not expensive and they're returnable No more rattles and tightens the bass significantly.