Vibration dampening

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.

This is an interesting discussion. I know a sound engineer from college and recently met a manufacturer of sound isolation gizmos. They had some interesting insights.

For most components, wool balls provide the best vibration buffer. Wool diffuses the vibrations randomly. It has no sympathetic frequencies and transmits a lower amount of vibration than any other substance. I buy wool dryer balls and cut them in half to put underneath my CD player and other sources with mechanics. Works a charm.

For heavier things like speakers, Steve recommends wood hemispheres. A half globe has exactly the same amount of contact with the ground or the gear as a spike - one point. Generally, a sphere spreads the vibration more. Wood is also better than most metals at dampening vibrations. (They can also be painted or stained to match your speakers.) Steve chuckled at the design of most audio gear solutions. He said most spikes and springs are like putting your equipment on tuning forks. Any isolation will help matters, but sympathetic resonances from "tuning fork" arrangements might create unwanted peaks and dips.

Attached are a photo of my CD player on its half balls, a picture of the 10 dollar box of wool dryer balls I bought at the grocery store to make them, and a 5 dollar bag of wooden hemispheres I bought at a craft store. Woolen Ball Supports  Dryer Balls Wooden Hemispheres

Of course, YMMV. Acoustics is a situation best addressed by experts, as each room and system produces different patterns and distribution of sounds and frequencies.Some rooms match your system marvelously, some not so much. Given that most of us cannot rigorously measure the sound in our rooms and determine with professional certainty what solutions will work, I use Jordan’s razor - start with the simplest, cheapest effective solution and see whether that does enough. As all audiophiles know, there is always someone looking to sell you an expensive solution based on little more than knowing you have a problem to solve. Which is not to say that you might not need an expensive solution, just that you might not need an expensive solution. There are a million ways to shield components. If vibrations annoy you, try until you smooth them out.