What is the perferred material for a TT shelf?

That would depend upon the TT you are using.

Sans that info any replies are clueless.

You guys have me thinking that I may have chosen the wrong material, or perhaps the wrong decoupling method.  Am I better off with a large set of spikes under my acrylic than I am with the Herbies Audio Lab Grungebuster Dots?

I have the Michell Gyrodec which is basically a Gyro on an acrylic platform decoupled with aluminum spikes and aluminum cones as feet on the underside.  Seeing as how Michell chose cones under their acrylic, I'd say that is a good place to start as any.  If it doesn't work out, as least it's not too expensive.

If your table is a non-suspended design: Two shelves of Baltic Birch Plywood separated by a slightly inflated bicycle inner tube, with the table sitting on three roller bearings arranged in an equilateral triangle. The most effective isolation short of a Minus K or similar platform (which cost in excess of $2000).

I purchased a Jean Nantais Lenco Reference Mk III a year ago.  It's a suspension-less design.  Jean recommended a 1/2" slap of marble laminated to a 3/8" think slab of acrylic that is the top layer.  I put Herbie footers under this platform to isolate between my heavy Merrill Stable Table stand.  Works great to my ears. I used maple remnants (beautiful ivory color, all polished and finished off) purchased from a local floor and tile shop - $200, and the acrylic from my local picture framing store - $150.  Cost effective, relatively speaking.