Subwoofer Footing - Connect or Isolate?

You can not isolate a subwoofer from a "springy" floor. It will shake/resonate from the bass regardless. The solution to a "springy" floor is to fix the floor which can be easily done with lally columns and a header. Once the resonance frequency of the floor is above the subwoofers highest frequency you are in business. 
People imagine all these EVIL gremlins lurking around corners. Lay instinct run amok. 
Any shaking, movement or vibration in any loudspeaker is distortion. With subs this is viciously true. The failing of most subwoofers is the enclosure. It is extremely hard to contain that vibrating monster in a cost effective manner.

The last thing you want to do is put a subwoofer on springs. At some low frequency it will start shaking. Vibration/shaking in any speaker equals distortion. Just put your hand on the sub while playing an organ piece or better yet run a sine wave sweep and you can find all the resonance points. Keeping a subwoofer from shaking is not easy. Certainly, mass helps as does containment. In the home environment spiking it to a concrete floor would be best followed by spiking it to a wooden floor. The wooden floor's resonance point will hopefully and usually be above the subwoofer's range. Even so the floor is going to resonate regardless from airborne excitation. New designs Like Magico's big subs are using a method of force cancelation. They put drivers in opposite ends of the enclosure running in phase with each other thus their Newtonian forces cancel. This effectively stops the enclosure from shaking but does not stop cabinet resonances, another issue all together. Magico does it by building a battleship enclosure.