Great creative experiment. It was worth a try otherwise we'd never learn anything.

I worked on industrial machines in my day gig and we were always trying to kill vibration. While soft isolating pads often helped, stiffening the mounts would also help in some circumstances. The reason we found after a lot of trial and error was that peak resonant frequencies could be moved around by making a system heavier - like rigidly locking it to another heavy object, or making a flexible thing stiffer. This is because the resonant frequency of a system is the square root of the stiffness divided by the mass.

If you can move the numbers away from the frequency that they're being driven at like your 3000rpm point you can often diminish the overall vibration. It's kind of like pushing a kid in a swing at the wrong rhythm or trying to get a car to bounce by pushing it up and down at the wrong rhythm. So sometimes I've found you can get a surprise by stiffening or making something heavier which is counter intuitive. Damping is only one of the knobs we can turn down.

That being said, I killed a lot of vibes in my SD by turning in the rack damper screw (yes, not only idle but smoother highway cruising) and doing a valve adjustment. A big one was tightening my tranny mount too. You've probably done all that. The W126 has a different insulation setup anyway.

Way to go on the attempt. A negative result is still knowledge.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles