Hello Agargara and welcome to the forum. I personally prefer to use a mouth piece than a tube, actually I don’t like the tube at all. I sometimes use the long extension tube on my Hammond melodeons, they provide a little extra room for air storage than straight into the melodica with just a mouth piece, this helps when playing and holding long notes. It really depends on the player and what feels best for him or her. Even on my Vibrandoneon that uses an s curved tube, I designed and had a special mouth piece receiver made that would hold a French horn mouth piece. My Eolina uses a s curved tube with a duck bil type mouth piece. I really do not like this and I am working on a design for that one as well. I play the melodica as a wind instrument and not like a piano, by the way Been playing piano since 1968 and melodica almost as long. I used to use a long tube taped to my mic stand and would sit the melodica on top of my keyboard so I could play one hand on my Fender Rhoads and lead on my melodica.
Monsters of Melodica