MDF or Plywood for best sound in Hi-Fi rack ?

In my view the jury is still out on Maple butcherblock vs plywood, for the simple reason that I have not yet seen the results of any in depth experimentation with high quality plywood. There are some very interesting plywoods out there. For example, Finnish company UPM-Kymmene makes an extremely high grade Finnish Birch (not generic Baltic) product. The mechanically most stable type is the one rated for exterior, which can be recognize by the presence of thin black lines of glue between the layers of veneer. The thickest sheets normally imported in the US are 1 and 1/8 inch thick, but 2 inch thick sheets can be potentially sourced from the manufacturer in Finland. Blocks of the material are so stable that they can be precision machined and carved for prototyping of mechanical parts by tool&Die designers. I have experimentally worked on some samples. Progressive power sanding down to approx 3200 grit yields a highly polished seemless surface both on face and edge.

If I recall correctly, a 4ft x 8ft sheet of the max thickness costs $120. Such a sheet is made from 20 layers of extremely hard Finnish Birch veneer. Conversely, the same thickness sheets classified as Baltic Birch is made from approx 12 layers.

Constructing shelves or platforms by gluing 2 or three layers of such UPM Finnish material is likely to yield an amazingly stable surface, likely devoid of resonance at audible frequencies. Whether such a shelf is inferior or superior to a similarly thick maple butcherblock unit has of course to be demonstrated. It is worth pointing out that one of the reasons that Maple is used in platforms and shelvings is the romantic notion of its inherent musicality, as it is almost universally the choice material from which the solid wood back of the best violins, violas and cellos are constructed. However, in a musical instrument, the back is part of a mechanical sound amplifier, in which the wood needs to vibrate to generate music. Not so in our isolation platforms, which we want largely to sound as dead as doornails.

UPM Finnish plywood can be sourced in the US from

Plywood & Doors
866. 738. 7265
Speak to Rod.

Please note that minimum order is $300, which means 3 sheets. From three sheets you can build a sizeable number of shelves and platforms.

Pauly, you have made some interesting experiments. Where your listening test blind tests?

Unfortunately I have only a small hand-sized sample of 1 inch thick UPM Finnish plywood. Not enough for true experimentation.

Here are some further thoughts:

If indeed the benefit of maple were transmissive rather than isolative, you may want to try a single large solid board, which we would expect to be more transmissive than a butcherblock, which is instead meant to break up vibrations and stresses to an extent. The back of string instruments is constructed from a single solid board, or two at the most.
You may also try to experiment with thin boards--one inch or even less, and see what the more flexible body has on sound.
After all, a company represented by Virtual Dynamics manufactures platforms and racks made from brass castings, that ring like church bells.

It is also worth pointing out that the original choice of maple by the old masters was partially a matter of necessity rather than of true exhaustive search. Maple being a rather common hard wood readily sourceable in most European sub-alpine and trans Alpine regions where luthiers resided and worked. At most, the wood was imported from neighbouring regions.
in the last few centuries, Luthiers having remained a conservative lot, never truly experimented too much with more exotic timber for their resonators.
Now days there are so many more very hard and relatively inexpensive woods to experiment with, even for 'isolation' platforms: Ype and Lyptus, just for example.
Have you ever tried to play with them?