@perkri I watched a bunch of Real World Audio's videos on Youtube. He's obsessed with single driver speakers using active cabinets (voigt pipes, folded voigt pipes, transmission lines).
Paraphrasing:
Real World Audio actually uses the violin analogy to describe active cabinets. Violins are made of the lightest, most rigid wood because you want the cabinet to resonate to amplify the sound. Wood with a natural grain is preferred but plywood is acceptable and much cheaper. MDF is bad because it has no natural grain. These cabinets are inherently harder to build because you have to fine tune the cabinet by slowly removing material and using bracing.
Slate is an intriguing choice because it has laminar structure like wood but is rigid and hard like stone. So is the Fischer speaker using a dead cabinet (MDF/granite) or an active cabinet (wood)?