Thiel Owners

Expansive grout is a waterproof cement product with adhesives, designed to expand in a crack to seal it. It may be called foundation crack sealer, wet-stop or similar.
Those original CS7 baffles were portland cement concrete, which continues to shrink over time. (Its first cure cycle is 7 years, when its shrinkage is nearly over.) Grouts add gypsum and/or other expansive masonry ingredients to achieve various controlled expansions. I don't know what exact product Thiel (or the nearby repair shop) used. But a millwright or masonry contractor should point you in the right direction.    Disclaimer: After my time at Thiel

The original Phase Linear amps from the 1970s were nick-named "Flames Linear". We had one. It did.

@thoft, Yes, I’m serious.

Has anyone ever had any problems with the lighter veneer on their Thiels? Mainly referring to a cup stain or something else and then tried to get it out by light sanding etc. 
@thoft did you do anything with your 3.5 on the outsides or was it just driver replacement. I really don’t want to have to reveneer speakers if it’s a light stain, but I’m thinking a little sanding will be needed, carefully. 

I googled expansive grout and all the top results were for non-explosive demolition grout.

That can’t be right.