Speakers on carpet over concrete

I have wall-to-wall carpet in my listening room, with stand mount speakers; I found a big difference, especially in the bass, between using spikes and not using them. I think there is a thick layer of concrete between floors of my apartment building.

Yes, it matters.  You want to prevent the speaker from rocking from the motion of the woofer, causing some Doppler shift. Of course this depends a lot on the geometry and weight of the speaker.  If a light touch can rock it, at all, it needs spikes.

My experiences.

Poured concrete basement, pad, carpet.

1. Vandersteen 2ce Sig IIs. Had spikes, tried IsoAcoustics Gaia's. The Gaia's really sucked the life out of the speaker especially the bass. Now let me say that I did hear a benefit with using IsoAcoustics Oreas with the integrated amplifier that I was using at the time, so I do have an open mind on decoupling. Anyway, I returned the Gaia's and went back to spikes on the Vandersteen's. The sound immediately was back to good.

2. Wilson WATT/Puppy 6. I have the stock spikes on these and have no issue with the sound. Not tempted to try IsoAcoustics on these at the moment.

Anyway, just my experiences.

With drivers out of phase or in breakup a bit of spring induced Doppler discoid the least of a listener worries….

Of course this depends a lot on the geometry and weight of the speaker.  If a light touch can rock it, at all, it needs spikes.

This is a good point.  With any of the recommendations here the weight, structure, dimensions, floor coverings, and other factors can all affect the success of a particular product.  My stand mounts are 180 pounds each, including the very rigid stands. They sit on an industrial type carpet over a dense pad over concrete on-grade. The solutions that I like for my situation may not work well for everyone.