All of the above.

Putting down a concrete pad works because it forces the vibration-causing unit to move the mass of the concrete. More mass = less movement.

Putting down padding eliminates or reduces frequencies in the range related to the mat density and transmits other frequencies (a sort of band-pass filter).

Springs act like padding but on a different frequency range.

John Doe devised a simple, elegant (and even sexy?) solution -- used breast implant gel packs to isolate vibrations from a turntable. Hydraulic dampers are extremely effective and can be made tune-able. There are commercial designs that combine springs and fluids -- on cars we call them, "shock-absorbers", but there are lots of designs for different functions.

I'm betting the problem you're looking to solve is 60 hz hum. I'd look for something hydraulic that attenuates the frequencies that cause the most problem.

First-pass guess.

B