spikes under a subwoofer ?

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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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

No one right answer. You sue the method that reduces the cabinet movement, simply put. THAT depends on the floor. For instance, my C4's with spikes firmly into the concrete floor simply blow away the speakers sitting on the carpet. ANY movement kills higher frequency doopler distortion, too, as the tweeter moves so little relative to a big old speaker rocking around. For my floor, the spikes rule!

I have DD10+ subs that sit right on the floor (odd) and I'm still wondering about that. Sure, bass is a LONG wavelength from a sub, so technically it is less easy to hear the doopler movement and yes, bass has a lot of distortion recorded-in, or produced by the driver. But, the C4's low-end resolution improved substantially with spikes into a concrete (not wobbly OSB subfloor)floor.

I do, and don't, buy that subs need to be on the floor with modern digital EQ. And, in my case, the room is 40 feet long so I have TOO much below 30 Hz! So again, it's the "system" that matters. Fix yours and don't just be someone else's stereo in your different sounding house.