1. Resonance frequency is the frequency in which a material vibrates the most. Isolation (decoupling) will typically worsen vibrations at and near the resonance frequency, but higher frequencies will improve. That's why you want the resonance to be as low as possible, preferably outside the audible range.
2. All tables (and everything) has a natural frequency; and maybe even more than one, if the table is made of different parts or materials. I've only seen some manufacturers publish this, and it is always dependant on the load or mass on top of it. Every system has an optimal load, and if it's less than that, the resonant frequency will be too high and might make the problem worse. If the spring is overloaded, it will become useless.
3. Don't quote me on this, but I believe amplitude absorption depends on deflection of the spring and the damping factor. Usually audiophile products don't publish these (maybe they don't have the equipment to test). But, for instance, the Minus-K mentioned above pushlishes their transmissibility curve, so do products like Green Glue, and manufacturers of springs, mounts, and dashpots, used in HVAC or auto applications.
