what is the theory behind maple stands and racks?

Maple isn't really all that hard, IMO. The vibrations that enter the platform are bounced around and some converted to heat within the wood. With most butcher block platforms there are many surface boundaries which effect the waves traveling through the wood. This causes even more scattering, but there is also attenuation as some of the energy is lost in the reflected wave. Eventually, things settle down.

The butcher block I used caused a smearing of the midrange that resulted in loss of detail and too much warmth. I may try a solid chunk sometime, but I suspect it will still be too warm for my preference.

There are many folks using big hunks of maple as shelves and I'm sure it does work in many applications. But obtaining better sound by using maple is no guarantee.

And those spikes that folks think act like some kind of mechanical diode, well it doesn't work that way. The spikes provide coupling which means vibrations can travel in both directions. They can be effective in some cases and detrimental in others. In general, spikes help emphasize bass and devices like rollerballs (non-compliant) help emphasize air and space around instruments. Products like Stillpoints are somewhere in between as they have both high-rigidity and some compliance. Personally, I have better luck getting the sound I want by using these kinds of products over what material the shelf is made from. YMMV, and all that.

Steven, FYIF, many baseball players have been using maple bats for years now. In fact, the maple bat seems to have taken over those bats made of ash.

How thick should this maple platform be?

Typically 2" - 4". Some application may work better with thinner platforms. For turntables and CD players for me, thicker has worked better.

By better I mean less exaggerated and more "natural" sounding highs, smoother but more detailed midrange, cleaner and better articulated bass. The whole presentation just sounds more coherent and better put together.

Has anyone tried any maple platforms under their speakers?
My old wood floors are a little soft, and I've been using small sheets of birch - 1/2inch thick- to set my speakers on. It did tighten the bass. Some of the maple platforms for speakers I've seen aren't cheap.