Random Effector moving object that does not move constrained dynamic spline

I have an issue that is bugging me. I have a dynamic spline with a constraint tag set to the end point on the spline and to a cube.
When you move the cube around the spline moves too, flapping in the breeze.
However, when I apply a random effector (in object deformation mode) as a child of the cube, the random movement of the cube is not carried through to the spline, which sits in the same place.

It seems to me that the random effector does not really move the cube object itself, but all the points at the same time, so the dynamic spline is not moving. Is that correct?

How do I make random movement of the cube move the spline?

Basically I am trying to animate a dancing pair of headphones, where the earpieces dance around to the beat of the music (sound effector strength linking to random effector stength in noise mode) and they drag their cables along with them in a pleasing natural way.

Any ideas?

R12