Marble to place speakers on?

It should extend about 2" larger than your speaker, on all sides. I would suggest that you try at least 3" thick 4 is better. Now for the best coupling, attach the marble to bottom of your speaker. Have your supplier drill 3 holes about 5/8" diameter in a triangle pattern that will be covered by your speaker. Don't forget to have them counter sunk so the heads of the bolts won't be exposed. Use these for a pattern to drill holes through the bottom of your speakers. Now use a 1/2" bolt, preferably fine thread through the marble & the bottom of your speaker. The speaker will then be totally coupled with the marble, with the maximum effect. The effects of the bolting together will be noticed by rock solid bass, better focus, added detail. Simple as this is, it really works. I have my Duntech Sovereigns bolted to a 4" slab of marble, polished black to match the finish on the speaker. The results are outstanding. Good Luck!

Sounds like we have a lot of interest here. It addition to my first post, may I suggest that there is no substitute for experimenting. As for sources, for thicker or larger pieces I found that monument works are great resources. Many times they have large stones that have been broken, & are willing to sell them at very reasonable prices. Cabinet installers that use granite or marble tops are a good source for the thinner pieces, ¾” or 1”. You may also check out tile supply houses. What I usually look for is damaged or broken pieces & them let the supplier cut them in sizes I can use. These stones can be used to fine-tune most systems.
I have found that if I use larger pieces with components sitting on top of them that the spiked feet work very well between the component & the stone. Again experiment with what sounds best; there is definitely a difference in each method. For speakers, bolting the slab to the bottom couples the stone to the speaker, virtually making the stone a part of the enclosure. If the stone is not attached I would suggest trying feet between the speaker & the stone with a large piece of stone resting on top of the speaker. This adds to the coupling affect. Yes, by attaching the stone to the speaker you eliminate the need for spikes on the bottom of the stone, making it much easier to move or adjust your speakers.
I have also had great success using a partially inflated 12” inner tube under the stone for components other than speakers. Here again, experiment. I have found the affects of using stone & inner tubes to make great differences in what we hear & at a very reasonable cost. GOOD LUCK! & HAVE FUN!