Vibration dampening

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.

@boxcarman - I use roomservice equipment vibration protectors on all of my equipment. I started with my sub and the difference was night and day. The bass is so much cleaner and tighter. I highly recommend reading the science behind the EVPs. 
 

https://avroomservice.com/evp-2/

A subwoofer is also a "speaker" (essentially), i.e., the same benefits one may reap from isolating a potent speaker shall also apply to a sub in a hifi application...unless the goal is to initiate boomerman mode, crack the drywall, bring the ceiling down, etc... like the hometheater dudes with a platoon of 18 inch subs.

Isoacoustics makes some good stuff for sub isolation that's cheaper.

If you want to spend more, Acoustic Fields can hook you up.