Vibration dampening

In talking about the spring pods to isolate, I actually put foam ear plugs inside the springs to act as a damper on the bounce and run one or two less springs in the setup. Effective loading without the wiggle and jiggle of springs and sound seems a little better overall.

And yes, pulling your sub away from the wall a little helps a ton to smooth out the response.

-Lloyd

I second the Auralex SubDude II..I used the V1 in a home theater for years with excellent results & still use SpeakerDudes in my bedroom system...

Symposium Svelteshelf with Maglev footers from Aliexpress

OP - "I am using spikes on thin carpet with concrete underneath."

It seems that you are 'coupling' your sub to the floor (and secondarily, the walls), not isolating the sub (which is quite different).  The aforementioned suggestions of using springs may work well.  I use inexpensive elastomer-cork-elastomer squares to transform the vibration into heat - isolation, not coupling.  

Give each a try... You are likely to be pleased with the sonic results.