Out on the tiles...with speaker spikes.

I have a beautiful black tile floor. I use Walker 1/2 inch discs to protect it. A bit more expensive than the other suggestions, but definitely worth it.

If your room is "on the slab" (concrete) you probably don't need spikes. From what I've read, spikes are best used on floaty, loose (i.e. wooden) floors or to provide acoustic isolation for us apartment "dwellers".

Linn used to make little feet for use underneath spikes, but I can't for the life of me remember what they are called. One of my friends uses them and they work very well. They seem to keep the speakers very stable. I would think you could pick up a pair for about 50 bucks.

If you've got the patience and access to the right equipment (I do!), you can cut aluminum round stock to the thickness you require, then countersink (a standard drill bit will work just as well) to the depth you want. The aluminum is soft, and easy to work with. Probably less abusive on your tile than a coin would be also. Then go to Home Depot or Lowe's, and purchase some felt dots to stick underneath. Although I much prefer the acoustics of a dead floor-- Carpet, with a heavy pad beneath.

Apc-d coupling discs from Starsound will give you the most improvement in sound. Lak is correct these work perfectly on all types of wood or stone products and they are an extension of The Audiopoint so they do improve energy transfer.. Its a combination of material [ brass ] and the geometric ratio by design..