The Grand Soloist Series has been producing big, fat marimba tones since 1990 under a different model name. They all have a rubber core plus an additional layer of latex -- in other words, all rubber, all the way through. They are wound medium-tight with an extremely durable proprietary synthetic yarn that produces clear articulation even at pianissimo. They are wound at a steep angle, producing a nearly round shape. Now available in four hardnesses, as well as in graduated sets.