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

I realize you are talking about a Velodyne, not a REL, but my experience with my REL is as follows:

Sumiko advised me AGAINST using spikes under the REL because it would change the distance that the sub "sees" from the bottom of the cabinet to the floor. I don't know about the Velodyne, but with the REL, since the spikes would be screwed into the existing feet, this would obviously be the case.

On Sumiko's advise, I bought a one inch thick slab of sandstone about four or five inches larger around than the sub, and placed the sub on it. The improvement was immediately obvious: much tighter and seemingly slightly more extended lower response. The somewhat rubbery sound (compared to my main's speakers') was reduced and integration was improved. The weight of the sandstone (considerable) apparently does a good job of coupling the sub through my carpeting to the floor.

Good luck.

We have found the same problem Dinos describes above. We have found that spikes into a wood floor; even through carpet does indeed mess up the midrange etc. We have found that something simple and cheap under even stock spikes make a huge difference in the overall sound. Yes even coins placed under the spikes help. This is not as big of a deal when speakers were on concrete slab floors.

I think that you're going to get BIG differences if the floor is suspended or not when using spikes. Directly coupling it to a suspended floor with spikes can REALLY excite the floor, especially when we are talking about a downloaded subwoofer. As mentioned, driver to floor distance can also affect the loading characteristics of said speaker too.

One trick that you can use is to spike the speaker cabinet and then place that on top of a board or large flat surface. The surface should be slightly larger than the cabinet. The panel lies directly on top of the carpet, which spreads the load out on the floor somewhat. This rigidly couples the cabinet to the board while the board is somewhat isolated from the floor by the carpet and padding. You still get the correct loading for proper driver output due to the panels' flat and smooth surface AND minimize floor excitation due to the slight carpet / pad insulating effect. Playing with the type of and mass of the panel and adding weight via mass loading can be used to "fine tune" the results somewhat. I've found that adding too much weight to the cabinet will tend to deaden the bass and make it slower though, so be careful. Just a thought... Sean
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Perhaps I should add that the HGS series subs are front firing.