Ofir, as I have mentioned here on many posts, not all melodicas are going to sound and perform good on all types of music. I have the Pro 36 and the Solist and I have only used them on recording a few times simply because I have not found the piece I want to use them on that I felt the tone was right. I have heard the Yamaha on many recording and videos and they all sound different based on the PLAYER not the melodica. I recently used my Clavietta for a new recording for the Monsters of Melodica which I had not ever used before simply because it was the best fit for the piece we were recording. I did not even use my Vibrandoneon because it did not fit the tone we were looking for.
on another note:
I am still wondering what you mean by a Flat sound (less rich) when playing loud? and the difference between responsiveness and richness? can you elaborate on that a bit if you can.