Thiel Owners

Max - I use Delignit / panzerholtz in piano restoration work - and love it.The earliest Thiels from 1975 to 1985, up to the CS3.5 used various products. The 01 and 02 were medium density particle board. The 03 and 03a were Baltic Birch (actually the higher quality FinPly). The CS2, 3 and 3.5 were high density particle board walls with high density particle board baffles. The model 2 and 3 had internal spruce struts inside the cabinets. I consider all these models to be of superior materials and outcome than the later MDF cabinets.
MDF was chosen for its consistency, universality of sourcing, flatness for laminating and gluing, and machinibility. With well engineered internal shelf braces, it was adequate for the job. Note that Thiel was producing extremely complex cabinets in a production environment with no room for anything to go wrong, and needing a universal material for feasible stock management. 1" MDF filled that bill. We were never a boutique, cost no object shop. Our finished products sold for sometimes a small fraction of what some manufactures lavished on their cabinets alone.
There are no CAD / manufacturing drawings available; but that may change in the future. I believe a more cost-effective retrospective approach might be to experiment with Panzerholtz, etc. as bracing material to quiet any cabinet wall resonances that can be identified. I am adding strategic long-grain spruce cross-braces to settle the enclosures, and have chosen torrified bamboo for bracing as a very high performance / cost effective candidate.

In the mid to late 80s, I experimented with various Corian-type products as well as honeycomb substrates with technical skins, curved panels and so forth. So much cost was added that we stayed away from them. We went to cast baffles machined with abrasive diamond tooling as an effective solution to affordable extreme rigidity It would have been very easy to double our manufacturing costs with multipliers applied for retail prices, and stray from our intended niche of phenomenal high value. Cost effectiveness was one of Thiel's evaluation lenses.

By the way, the CS3.7 / 2.7 returned to one of those 1980 experiments. The curved side panels are 17 ply birch laminates, using the material and its geometry for added rigidity and waveform propagation.

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.