[Ofir]

Melodica-Me, from my point of view, I must disagree with you on several things.

First, I’m a big believer of 1-size-fit-all when it comes to acoustic instruments.
Although every instrument has its inner soul, a good instrument must comply with the musician’s needs in a reasonable sound-range.
Same way as I won’t require a different piano for each piece on my concerts (the festival owner would kick me as far as I can imagine..), I would expect the same melodica to fully serve me for whichever piece I want a melodica in.
However, If I want a distorted electric guitar then I will neither expect any piano nor melodica to fit. But a reasonable range must exist, and it must be large enough.

Second, I’m not sure that your impression of the Yamaha is accurate, as you only heard it on recordings.
You know very well that the recording and mixing process has a major role in delivering the sound of an instrument, and I’m sure that attending in front all of these players, you would get a much smaller variety.
I’m well aware of the ability of the musician to control the sound of his instrument. I do it very well on my piano, and I think that I’m doing it pretty well on the melodica, trying to check its inherent variety. However, I believe that the Yamaha P37D’s sound range is yet not as rich as I would expect my instrument to be for professional usage. Not enough harmonies, sound it too ‘small’.

As for being flat when loud – I wrote that to Daren on a different post:

Melodica reeds – harmonica or accordion style?

As for responsiveness vs richness – my experience with melodicas is yet very limited, but my understanding in physics and acoustics is not. I was referring to my modest experience with the Yamaha vs Swan, and extended that with what Lowboy wrote on the difference between the P37D and the Piano 36 (on the same post I linked here).
You may add to that your experience with the difference between thin and thick reeds, and I believe that it will match my description.