My experience is that decoupling is the best approach for both speakers and subwoofers. Stop the physical vibrations of the speaker or subwoofer cabinet from ever reaching the surface upon which they sit. That way, you are only hearing the sound produced by the speaker; not the sound of the speaker plus the sounds caused by the vibrations of the stand/floor/shelf/what-have-you.
So in the case of subwoofers, I advise putting a decoupling device inbetween the "feet" of the subwoofer and the floor. A really thick carpet pad can act as a decoupling device. But one issue are the "feet" on the bottom of most subwoofers. When you have all of the weight of the subwoofer supported only by 4 points, those 4 points have a great deal of downward force and can easily "sink" into most carpet pads far enough that they actually couple the subwoofer to the floor.
A similar thing happens with "spikes" for tower speakers or speaker stands. The speakers are actually directly coupled to the subfloor beneath the carpet.
There are a few things at play here. If the spikes do their job perfectly, they essentially make the speaker and the floor into one large, very massive structure. That works to make the combination more inert, simply by virtue of creating a very massive structure. With such high mass, it takes a great deal of energy to shake that structure, so in that sense, a perfect spike makes the speaker more inert.
But it is VERY easy to have a less than perfect spike. If the spike fails to completely join the speaker to the floor structure, you just end up with 4 sources of very strong vibrations. ANY shaking of the speaker at all gets focused into just those 4 points and if they manage to vibrate at all, those vibrations get transmitted into the floor and you get distortion as a result.
I say to avoid that can of worms. Decouple the speaker and you avoid this possibility entirely. If you are putting bookshelf speakers onto a stand, I suggest decoupling the speaker from the stand using Auralex MoPads or a similar decoupling device. If you are using tower speakers, I suggest putting SubDudes underneath the towers so that their spikes are NOT in contact with the floor at all!
What you will often find - if you are used to the sound of a subwoofer or speaker when it was coupled to the floor - hearing it after it has been decoupled, the bass will sound quieter and less forceful. After a bit of time though, you will realize that all that is "missing" is the vibrations of the room itself! While having the entire room shake along with the bass can be exciting and tactile, it is actually just distortion. All of that room shaking was never part of the original recording. It is a byproduct of physical shaking and that is distortion.
With speakers and subwoofers properly decoupled, you hear only the output of the speaker. So bass will seem quieter and there will be less tactile sensation of the room shaking, but you will gain an enormous amount of clarity, detail and accuracy.
