Clean single notes . Sloppy play is an enemy of good tone. Playing in tune . Also, the tuning system used, such as equal temperament or just intonation or some compromise between the two, affects the tonal characteristics of passages and chords. Breathing from the diaphragm helps produce a big fat full tone.
Resonance works with breathing to produce a loud or complex full bodied tone.
Effective use of vibrato works with the rhythm of the music to add variety and life to the sound of a note. Consistency of vibrato through the different note types can be a key to maintaining a cohesive tone through a passage, and mask the playing techniques used to create the note. Consistency among note types in a passage contributes to the sense of a controlled and motivated tone. Open airways , including the mouth and throat help get the thick round tone. Pinching of the air stream makes a note weaker and thinner and less confident.