Hello, from Milwaukee, Wisconsin, USA, home of the SPAH Convention 2007. I think it was Zombor who asked readers of this chat group if anyone has tried attaching tubes to reed plates, or tried reeds in tubes, to produce amplified reed vibration when activated by our inhaled or exhaled breath over the harmonica reeds.
Well, just take a look at the Hohner claviola. It is not amouth harmonica anymore, being more similar to a melodica. But it has dedicated tubes for each reed, and the tubes themselves can be tuned. The sound is quite unique.
As usual, Pat Missin has great info on this on his website: http://www.patmissin.com/history/claviola.html
Cheers, Fernando