In my experience, the more connected I am with the melodica, the more control and satisfaction I get from playing it.
The closer my mouth is to the reeds, the better control I think I have.
In addition, you must think not in terms of your breath and the instrument as separate elements, but rather one big integrated air chamber starting at the bottom of your lungs through to the exit hole of the note you are playing. Everything along the way (lungs, diaphragm, throat, mouth shape, tongue position, breath control, mouthpiece/tube, length and volume of the air chamber in the instrument, and more) influences the sound and expressiveness you can achieve. It is all one big resonating chamber.
Sing or hum as you blow into your melodica and hear and feel what happens. You can begin to set up resonances.
Some melodicas seem to have very short, small-volume air chambers. Others, relatively speaking, have very long and voluminous air chambers.
For reasons that are too long to explain here, and because the regulars have heard it too many times, I use a mouthpiece that looks like this.
Image of Mouthpiece
It is as short as possible but allows some movement of the instrument. Like Melodica-Me, I am also working on a custom mouthpiece and will describe it when it is done. Been working on it for a year, so don’t hold your breath.
So which is better, tube or mouthpiece? Short is best in my mind. If I could, I would blow directly into the hole and the giant air channel/chamber of the melodicas I play. But that limits me moving the melodica around. So for now, I use the above mouthpiece/tube combo.