spikes under a subwoofer ?

You may want to decouple the sub from the floor,and get
an even better result! Most people believe in old truth,
without understanding too much about this complex matters.
Vibration control is indeed a complex subject, but according to an inverstigation made here in Sweden, spikes often endsup with an inferior result, because when you couple the sub to the floor,AND the floor is prone to resonate!These are unwanted contributions to the sound that you want! There are decoupling devices that prevent this from happening(altough having a soft carpet to place the sub on makes the whole thing more complicated).

For my part, using soft non-elastic feets called the"SD-fot"
that are made in different"densities", for loudspeakers or subs with a specific weight, and which makes it almost impossible for the transducer, to feed the vibrations to the floor,has in my opinion cleared up the bass as well as the the other registers.
This metod has been accepted here in Sweden, even among the
rather conservative magazines. One test one of them made,
was simply to place a coin on end at the top of the loudspeaker, while playing some bass-notes;without the SD-feet, the coin fell, with the SD-feet it stood up.
This decoupling tecnique was accepted several years ago,
when it came to tone-arms an pick-ups(warped records).
Time to rethink some loudspeaker-truths maybe?One italian reviewer of these devices, clearly misunderstood the fact that with the damping feet under a loudspeakers, you can swing them a bit
back and forth, by hand;it might feel as it they are a bit unstable,BUT that is intentional! That resonance is to low in frequency(say 8Hz) to be engaged by the woofer.The loudspeakers enclosures will stand more still with the soft feet Not too uncommon with less educated reviewers
and conclusions that are wrong,eh?

I feel that I may get a lot of opposition, for writing negatively about the "holy" spikes.But please try to be open-minded, there may be another truth!
Regards
Håkan

Have a look at this page: www.sonicdesign.se/sdfeet.html
Quite interesting, and to my knowledge, you can excite
resonances in the floor even if it´s made of concrete!
I have tested the SD-feet just in this case,(and at different places) and the improvment in sound quality was remarkable!
The Danish loudspeaker, CDP and amplifier- company Holfi is marketing simular soft feet, they also claim that floor and loudspeaker cabinet vibration is significantly lower with their softfeet(some thousand times lower vibrations, if I remember correct).
Of course,I have no commercial interest in either of these
companies.
You have to weight the front- and back-side of your loudspeaker or sub, to get the right stiffness in the feets.I think there are feets up to a load of about 450 Lb. They are made of a material that doesn´t have closed cells,therefore the feet wouldn´t collapse with time.
I can´t say what the prices for the SD-feet are at the moment(set of eight of them),but I believe that you should
be able to get a set at about $130 including shipment
to the US.

Regards
Håkan