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.

SVS sells their SoundPath sub isolation footers for only $50 and offers a generous risk-free trial so you can try’em at home. They are said to both improve bass and reduce transmission and get excellent reviews from customers who bought them. No brainer in my book. Best of luck.

Another Auralex Subdude user here.  Cleans up muddy bass that obscures midrange detail when flooring is problematic.  Everything is more refined and detailed top to bottom.   

key phrase:
when flooring is problematic

Great feedback everyone, thank you. As a low cost test I ordered some Nobsound springs, though something like a Subdude is definitely under consideration. I shall report back.