Subwoofer Footing - Connect or Isolate?

Spikes; definitely not springs.

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Yikes, I've never seen a place where they worked. There is vertical and horizontal, vibration control. There is also From and TO the source. It's  difficult to make points work on both plains and variance of the two..

Have you EVER seen any type of power plant that wasn't isolated. I've never seen one. You ever see one on points? I haven't!

LOL Kinda like steel wheels they never did catch on.. Maybe a BIG  dimple roller.

Every time it's a gummy puffer (soft mount) with a hole through the center and longer bolts with washers and springs, on either side of the mount sandwiched between the gummy puffers. You back up and slam it forward or forward and slam it in reverse, the spring compress, while the mounting plate is between silicone wafers. Pretty tough!!

You should see what happens to a typical 4 stroke diesel (5-15 liters) engine that loosed the integrity of a harmonic balancer. Break in half!

You can't make it solid enough, but you can make it soft enough..

Regards

I discovered early on that placing my Velodyne HGS-15s of Auralex SubDudes really improves their performance.  I do enjoy that subtle more felt than heard experience when a large pipe is played by the organist.

I just bought 4 Townshend Seismic pods for my 140 pound sub. I say vibration isolate your sub from the floor.

Using the pods is a relatively inexpensive way to completely isolate your sub.

I use Auralex Subdude platforms. Things sound more defined, controlled and right with them. 

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.