Violinist-composers tend to load up their music with sixths because the sixth is such a harmonically rich interval. It is simply loaded with overtones, some that can be heard, and some that can't really be heard distinctly. They can be felt though, by the person playing and the people who are listening. It is rare that a microphone can pick up the full array of overtones and difference tones. These are the things that give texture to the music and contribute to the personal quality of an individual player's sound.