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.