Slate vs Granite vs Maple?

You might consider having the rack designed to accommodate after market shelves like the Symposium's.

Research "Corian" It's completely non-resonant in the audio/audible bandpass. It's available in a bunch of thicknesses and colors: (http://www2.dupont.com/Surfaces/en_US/products/corian/corian_use_gallery.html)

Hmm, interesting... so since I have tubes, the best bet is probably granite. Maybe I'll try the combination. Rodman, thanks for the link to Corian, but I was under the impression that total non-resonance isn't necessarily good for TTs? I heard phenomenal things about Neuance shelves... are they out of business? I'd love to get one but I never see any on AG. How does Symposium compare with the Neuance?

All tubes and all maple here (much like this photo), and would not go any other way. Two other audio friends who also post here, Sbank and Sliptknot1, have followed the same path with great results in their tube-based systems.
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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.