Speaker placement directly on floor?

You want to spike them to the floor. Probably another member will know a source for spikes but all you need is to go to a hardware store and buy "dowel screws" in the correct thread size. Take the rubber things with you to be sure. Dowel screws are bolts that are half straight thread with a flat end and half tapered thread ending in a point.After that's done, take a water glass, turn it upside down and tape around the mouth to seal it to the floor and leave it 24 hours. If there's more than just a few drops of moisture in it you need a dehumidifier. Of course, this time of year there's not likely to be a problem, but there could be come summer.

I use Polycrystal collossial spikes under my PSB Silver-i
to great effect. You will definitely want to look at area rug on floor between you and speakers, as tile/concrete floor will cause unwanted signal reflections.

You can get quality spikes and matching discs at audiopoints.com

Forgot to mention that Polycrystal (and Audiopoints) can
come with 1/4 - 20 threads that screw directly into base
of PSB Gold-i. Audiopoints are brass, manf recommends slim
version for Golds. Polycrystal are also brass, but coated with polycrystal material.....for full effect go with the
collossial 2" spikes.

The seller also included some threaded spikes with the speakers, but I thought that those were used on rugs.
My floor is hard ceramic tile squares.

The threaded spikes are an inexpensive way to control acoustical, mechanical and electronic generated resonance. Some times other products such as Audio Points look and/or work better, thats a personal decision. You can also purchase the Audio Point Coupling Disc (APCD2) if you are concern about the small indentation made by the spike or Audio Point.

I think it would be a good/cheap idea to get a set of coupling discs to put between the spikes and your floor. They only cost about $6 each, would protect your floor and are (theoretically) better than using the rubber feet.