MDF sand box tweak - effective for DAC, pre-amp?

yes its a definite improvement, a preamp with tubes benefits more than a digital converter.

I use one under my TT VPI Jr and it is great. I also used them under my tube amps. Again with positive results. Little tweaks all add up to great results. I had a master cabinet maker make mine for $40 each. I painted them myself and they look and perform as well or better than the mass produced.

The only thing I would add to the above is to use Maple instead of MDF. All woods have a signature, maple, oak MDF etc. The top shelf, the one over the sand made out of maple or oak (solid, or glued like a butcher block)would make for a faster more alive sound than mdf. Besides if you stain it, it makes for a much more atractive material. Idealy make the whole thing out of wood, it will still be much cheaper than the comercially available ones.

Just my 2 cents. The original idea came from the folks at Shun Mook.

Good luck