Thiel Owners

tomthiel,

I appreciate the info and no surprise here that the other materials you mentioned yielded a better cabinet. 
I met a gentleman years ago who sourced material for Gibson. He was in his late 80's when I met him and I purchased AA/AAA Stika Spruce in widths up to 10" and thicknesses of 1.5". Grain is 25-30 per inch so this is stuff you would see on the front of their guitars and I have it in lumber sizes. 
I've entertained the idea of a cabinet with the spruce as it is a very stable wood and high strength to weight ratio with Delignit bracing.
Sanded and multiple coats of clear coat the wood is very beautiful in its natural state with no pigments added.

My thought was/is the cabinets in Thiel speakers was the weakest link and if this was ever pursued what would the end result be?

Max - I had never considered that Thiel's weak link was its cabinets. Let's put that idea on the back burner for now.
Regarding Spruce as a cabinet material - that has merit. Spruce is an extraordinary wood, landing it as the soundboard of choice for many musical instruments. My primary business after Thiel Audio has been producing tonewood for high-end instruments. Red Spruce (the holy grail of tonewoods) is my specialty. As a grown material it's hard to beat for stiffness to weight and so forth. But as a cabinet panel, it doesn't measure up all that well. It is a naturally resonant material, which is what you want to avoid in an enclosure. It is highly directional, being 7-10 times as stiff along the grain as across. Its micro-structure encourages transmission of sound through its thickness, another thing we're trying to avoid. And even though it is relatively stable for as a lumber species, it still expands and contracts enough with humidity changes that the cabinet would change characteristics through the year and eventually crack. So, all in, the idea doesn't check my boxes.
Among the materials science aspects of Thiel's cabinet walls, I chose a Canadian MDF made entirely of Spruce fiber, for the reasons you cite. We then laminated veneer on both sides with a structural adhesive, creating an I-beam effect with about double the stiffness of an unlaminated panel and more tha 1.5x that of an exterior-only veneered panel.

Dr. Toole and his proteges at the Canadian Research Institute looked at cabinet material candidates regarding stiffness to weight, stability and damping, and landed on MDF as the closest to ideal. PSB, et al made a case of MDF being "better" than most of the exotics out there. I wouldn't go that far. But putting cost into the equation tilts things significantly toward MDF being a pretty good substrate.
All that said, my present design work uses other materials to augment and upgrade the basic MDF cabinet.

Let me rephrase my previous statement. I don't think the Thiel cabinets are poorly engineered. My issue is MDF. Granted spruce fibered MDF is a cut above the other grades. 
Spruce bracing would seem more appropriate with Delignit framing/structure. 
I deal with delignit on a daily basis so very familiar with what it is capable of. 
I appreciate your input. 

Max - I consider Delignit to be a wonderful material and would be interested in what you know about it as a bracing material.

I have had some success adding cross struts of bamboo (in the league of Delignit) to the MDF shelf braces, which keep the struts from vibrational deformation - a nice pairing of attributes.

tomthiel,

I'm not a speaker builder so regarding Delignit as bracing I do not have that data.
I'm a US manufacturer in the motorsports industry and we are the authorized dealer for Delignit for motorsports in the US.
When a Formula! or Indy car hit over 200 mph the delignit holds up for the skids and bracing in these cars so I would have to think it would perform well with a moving speaker cabinet with nowhere near the velocity of a Formula 1 or Indy car. 
I'm game to experiment as I have the CNC flat panel routers to cut the Delignit into what ever shape it needs to be.