Pls pardon my ignorance: what kind of line variation do you need for music composition? Is it for the Italian dynamic markings? The notes on the staves?
You need different widths of lines, both in the dynamic marks and staff markings, but even in the notes, with the noteheads and beams being thicker and the stems being thin.
However, if this thread gives the impression that line variation can only be accomplished through flexing a nib, it is a poor source of information. Between a standard 'blob' nib and a flexible nib stand the various italic grinds, which give both thick and thin lines - precisely what is needed for traditional music manuscript. See my above response as well.