Isolation devices are not often so good ...
Springs are complex to tune and better used with damping load and way better when we use two set of springs one set UNDER and one set above the speaker under damping load ... the damping load may be tune with a few grams approximation by ears ...I did it ...
Coupling and decoupling work very well but ONLY with a sandwich mix of plates , I use oak, granite,sorbothane,bamboo, cork plates , all together with astounding results ... Cost : peanuts ...
Sorbothane alone, springs alone or metal feet alone are not so good at all ...These PARTIAL solutions are deceptive and hide the problem ...
Most people as it is the case with the acoustic control of a room or with the electrical high noise floor of a house , or with EMI interference , most people had no idea how vibrations/resonance plague all audio system because they never adress the problem really and at best bought some feet made of a singular material composition ...It is not enough and deceptive solution ...
When vibration are under control the impact is huge not small at all , including timbre experience, imaging and even soundstage in my experience ...