I recently put together a little 1950s-style mono-FM vacuum-tube
system. I didn't have a period speaker so I used a late-80s Kenwood
two-way sealed-box speaker I got at a garage sale for $3. I didn't
like the tubby sound, so I took the back off the speaker to see if
adding more fiberglass damping material might help. To my surprise,
there was none inside at all! I found some leftover attic insulation
in the garage and filled the box with it. That solved the problem.

Was damping material commonly left out of cheap speakers to add more
bass thump?

Brian