Sorry I’m so late to the discussion. I just saw this.
If I were in the same boat, the first thing I would do is go to s store that sells bricks/patio pavers/ blocks and look for the concrete formed, big, patio blocks. They usually have designs on the top, but you should be able to work around that. Or, look for blocks with a nice, flat underside.
These blocks usually come in two sizes. The regular size is about 18”x18”x2” and weigh about 23 lbs. The larger size blocks are 24”x24”x2” and weight about 42 lbs.
Get yourself a pair of those and experiment with the different ideas, If you find an idea that sounds good to you, then you can get fancy and buy some granite. Who knows, you might decide your speakers on top of concrete gives you the sound you desire.
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