Slate vs Granite vs Maple?

Non of the listed is very good - they all impose their sonic signature to everything. I have had success with Gingko products. Check their website. They are mighty nice people too.

I think in this hobby, there are no absolutes. What I mean is, one type of isolation may better suit a certain component, while another will suit the other component.

For TTs, I believe the best is still a combination of high mass (granite or equivalent) and a medium density material (maple or composites). Granite by itself rings. Maple/composites absorb this ringing. Maple by itself does not remove very low frequency feedback from full range speakers. However if you put them together, maple/composite cancels out the ringing, while the granite removes the low freq vibration.

I would not recommend putting a TT high up between the speakers as this affects the imaging. Placing it low and/or along the side wall is a better option.

One of the most impressive TT isolation boxes I've seen is Charlieboy's:

The box is made of solid maple pieces. Maple is very good for vibration control. The box is 2 inches thick. It measures 6" tall x 24" deep x by 32" wide. The box is screwed together then Ii caulked the inside so no sand could escape. The box itself weighs around 60lbs. Then i filled it almost to the top with 100lbs of fine washed beach sand that I bought at Lowes for about 4 bucks a 50lb bag. Then I had a 1" thick slab of granite cut to the inside demensions of the box.

You might be able to take some of these ideas and incorporate it into a larger audio rack.

A lot of great ideas here! Thanks, guys. Charlieboy's idea's intriguing, especially as it incorporates the granite slab within the maple box. I'll make sure to bounce these ideas off my carpenter. I guess no one's really a proponent of slate, though?

Granite sucks the life out of the music. I've used the granite slab under the solid state power supplies for my tube equipment and I don't like the results. A maple platform allows the harmonics to remain intact and the frequency balance is more even through the whole range from bass to treble.