I would recommend spikes. You want ridged coupling for speakers. If there are books (heavy ones) on the shelf this can also help. The shelf material is likely to be the biggest problem. If you can replace the shelves that the speakers are on with something that is non-resonant (nothing is really non-resonant, but wood is probably about the worst material). Veneered MDF would be preferable to solid wood. I have a set of bookshelf Epos speakers in a family room on Granite on tiptoes. Spikes in the speaker would be even better, but I haven't gone to that length yet. The granite works very well--you could probably get a couple of remnant pieces cut from your local kitchen counter top store to sit on top of your shelves and then use the tip toes or spikes.