I will not venture to say these work better or worse than a marble slab, but, you can get various kinds of floor protectors. These are available in a myriad of materials from lots of dealers. I use Tip-toes. The good news on these is that you can easily move the speakers to adjust toe-in, etc.
Granite Under Speakers Over A Wood Floor
I would Like to use something over my wood floor that would not scratch it like the spikes do. I remember seeing sonus farber speakers on granite blocks. Does anyone have any experience with this? How would this sound?