As harp-l's self-appointed, skeptical old curmudgeon, I hear the call of duty to bust another myth.
There is nothing about the acoustics of the harmonica that suggests that the covers play any role in the generation of the sound. The covers define the shape of the sound passage from the reeds to your ear. They might conceivably affect the coupling of the sound to the outside air as does the bell of a horn. They might have a slight effect on loudness but not tone.
Because the human ear has a logarithmic response, small changes of sound energy are imperceptible.
I predict that in a blind comparison of machine-blown diatonics, listeners could not distinguish between groups of harps with covers and no covers (much less covers having minor design differences) by the sound alone. I suggest that any effort to achieve nuances of tone (warm, bright, etc.) by means of cover design is wasted.
Vern Visit my harmonica website www.Hands-Free-Chromatic.7p.com