Hello, Robert Temple.
It's only an opinion, that all of my harmonicas
that are louder than their competitors, have one thing
in common:
special, "~" - curved covers. Not all ~ covers
seem to have the loudness factor.
On the ~ covers: view from left to right, with the
mouth placed on the left side of the ~ : this is
a side view of the harmonica covers.
The Hohner CX-12 gold cover; and the #1896 Hohner
Marine Band are louder than their competitors (to my
hearing-impaired ears).
Here are more opinions, not based on scientific tests,
but on personal experience:
If all other harmonica parts are of equal quality:
Thicker reed plates have a more air-tight chamber seal,
but don't seem to have any influence on the loudness.
Wood combs don't seem to have any influence on loudness
(compared to metal or plastic combs), but they do seem to
support a more smooth, mellow sound. Or, is that effect due
to different reed materials (alloys)?
Best Regards
John "Shake & Bake" Broecker
Sheik of Sussex, Earl of Cloves