My FR20’s are on a rug but I use Herbie’s giant decoupling sliders under them, largely so that they are at least somewhat moveable. I can’t claim to have done much in the way of listening tests with and without them. As mentioned, the Isoacoustic Gaias would seem to be another possibility. Here’s a link to the sliders:
I also have used the Herbies sliders for many years. My speakers are lighter than the FR20s, so I’m not sure just how they would work in your system (Herbie’s site probably has maximum weight info). But I’ve been happy with them and they do enable me to move the speakers while protecting the floors.
If you are planning on keeping the spikes on your speakers, I also highly recommend you give the Herbie’s decoupling sliders a listen.
They are very fairly priced and I think their design and purported efficacy makes sense; in that they should prevent vibrations from bouncing back up through the spikes and/or prevent vibrations originating outside of the speakers from traveling up through the spikes and into the speaker cabinets (at least that’s the claim/theory as I understand it).
No you leave the sliders under the spikes as that decouples the speaker from the floor. Another alternative is that they also come in a “studded” version that replaces the spikes altogether. I have used both types.