what is the theory behind maple stands and racks?

The trouble with oak is it is far too lofty and grab up all the light. Maples have been oppressed.

Go check out the 'boneyard' of your local granite/quartz/marble countertop fabricator. They'll have all kind of options for very little. It's only when they cut and finish the edges does it get spendy. I like quartz because it's a mineral filled resin that is very dense (25 lbs/ft2) for the 3cm and very well damped because of the resin + mineral composition. 

Guitars are different - you want them to resonate, but in a very controlled way, that's why the bodies are thin wood, usually very stiff, but no one would ever confuse rosewood with mahogany or maple used in the backs or sides. Spruce of varying species is preferred for the tops, with mahogany and maple used for specialty instruments, like a 'Hogtop blues guitar, or a Gibson Maple Jumbo with a midrange voiced to cut through the rest of the band. A neither sounds anything like a Martin Dreadnought. 

But I don't think  most people want to use vibration platforms as tone controls, so dense and non-resonant is the better way to go.

For some 20+ years in the middle of my speakers, but set back, I had a vertical Solid steel vertical rack with first the supplied MDF shelving, then over time various thick wooden cutting boards (John Boos, etc), and a couple exotic shelves made with violin wood. However, the height of the rack bugged me as I was convinced it was messing with the sound stage and imaging. A couple years ago I got the urge to go horizontal with maple (IMO) quartz/granite/glass, etc ring: YMMV, but it was way too expensive, but I got super lucky and found someone selling a DIY solid maple table for $200!. The top is ~ 50 x ~ 28 x2. Not only was it more visually appealing, but the both the imaging, sound stage and warmth that was missing from the vertical rack improved in a huge ways

See my Virtual System photos of both

hth