spikes under a subwoofer ?

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.

Good point : )

The effects mentioned above should still be similar but possibly not quite as drastic. Sean
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