Speakers on carpet over concrete

Decouple with individual springs, properly sized for the weight of your speakers or Herbies Giant Fat Gliders or Threaded Stud Gliders. Size individual springs by putting the actual load somewhere in the middle of the usable range, and account for any differences in weight distribution between the front and back of your speakers since the front can be heavier due to the weight of the drivers.

For a very low cost, cut squares of Owens Corning 703 or 705 rigid fiberglass, or use hockey pucks. The Owens Corning board is very similar to what is used in the AV RoomService EVPs.

If you want to pay more money look at Townshend, RevOpods, IsoAcoustics GAIA footers, or any number of other popular products. Some like Nobsound spring footers, which are relatively inexpensive.

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.