Hi BW77, I thought I would run with the efficiency of your approach:
"sounded different because the plate transmitted to other of the reeds on the same plate which possibly responded"
I wonder if that is possible really...*what* do you mean might be transmitted in such case?
Maybe incorrect, but reed plated instruments: melodeons, some early german concertinas, of which I have one, harmonicas and bandoneons, are very different in sound to instrument with a single shoe for one or two reeds. Some of this may be due to thickness of the plate, although I once had a bandoneon with very thick plates, some may be to do with the profiling: but I have, and I must admit with no research into the matter, thought the sound to be carried more easily along the metal, allowing it all to speak, and with its rigidity possibly vibrating others of the reeds in short waves along the reed so generating the distinctive sound. I once owned a maloedion collection, 27 instruments all up made prior to 1939, most had reed plates, and sounded like harmonicas, two had single plates of two reeds per plate, blow and draw: these instruments were in no way different to the others but in this regard sounded like accordions.
"as accordions got reeds put into chambers,.... the reeds became more mellow, and often louder"
Just some terminology nit-picking: The*reed* sound likely does not change but the *tone* outside the chamber/instrument:
" These cassotto chambers made the reeds more mellow,"
Again....the *reeds* are not affected. The cassotto influence acoustically probably is a combination of "formant resonance" and absorption of higher overtones.
There is possibly some debate about this, of the only two people I have ever spoken to about cassotto ('box' in Italian), one argued that the back pressure changed the shape of the reed and thereby changed the sound, the other that the cassotto absorbed the upper partials. The first said a cassotto, was a trial to tune because the reed's pitch changed considerably and unexpectantly when tuned on the bench and replaced in the instrument, and this was due to the 'back pressure'. Malcolm Clapp, although not one of these two tuners, may be able to help here as he'd tunes concertinas and accordians. I have just done a net search and again have come across both explanations.
"has anyone experimented with a concertina and noticed what difference the cupped hands held either side of the instruments makes: I believe it changes the sound"
Probably have many such experiments been carried out but reports seem to be few. Concertina maker George Jones is said to have tried increasing the volume underneath the endplate. That probably would work in a similar way as the said accordion cassette to achieve an "organ-like" sound..
I'm sorry I did not make the point clear, it was: when playing a concertina the hands are shaped like cups around the concertina, so in the few I have made there has been an attempt to accentuate this shape by curving the hand rest to further increase the cupping, and I like to kid myself it has a small effect on the tone by mellowing it.