There is natural resonance of the device versus the resonance of the device environment as it floats on a non-rigid mount. Both of those are on a platform that is vibrating, we presume, at different amplitudes based on the SPL and frequencies.
Not unlike running a sine and random vibe sweep of a piece of electronics for your car, plane, or tank. If the platform was to be your shaker table, then what happens to the device on the floating mount?
Adding the weight to the unit changes its natural frequency but you are still then floating it so now the forces are more about the floating device than the unit itself (assuming the weighted unit is in a non-coupling frequency with the sound, and it actually causes some sort of negative performance across its circuits).
Here's an opposite view, an accelerometer in your car's air bag sensing device must be as close to 1:1 transmissibility with the vehicle "frame" as possible so it gets as pure a deceleration signal it can to tell the bags to deploy. So, in this case, stiff mounting is desired. As long as the PCB in the sensing box is not floating, then the 1:1 is achievable. And we really don't care too much what the natural frequency of the sensing device is as long as it does not cause the accelerometer to misfire on bumps (and it won't due to axis's and severity[G's]).
Since we are not banging our stereos at 10 or 20 G's, presumed negative impacts are on a much lower level but nonetheless rapid cycles. Decoupling our gear from earth is beneficial for the micro-vibrations affecting our sensitive electronics which are not built with vibration in mind like an aerospace black box, for instance.
Of course we float our devices and then put on heavy cables, especially power cables, that carry vibrations back into the unit. And all of this making the presumption the vibrations cause negative sound and they may be enough for us to "hear".
I do isolate all my gear so each device can try and act as it was designed on the bench and given a chance to perform that way. Can I hear a difference? Not sure, but I like the tweaking aspect and avoid the math because it's a hobby and not the job. ha