This instrument is still experimental. It has its origins all the way back to Sax’s original patent, but only in the last few years has a real instrument become a possibility. It is pitched one octave below the Bass Saxophone and two octaves below the Tenor Saxophone.
Benedict Eppelsheim of Germany has produced a single Subcontrabass in traditional form. They also produce an instrument called a “Tubax” which is a very narrow bore saxophone. These Tubaxes are made in Contrabass and Subcontrabass range. There is also a Brazilian company working on producing a Subcontrabass, but they have yet to succeed in making an instrument that fulfills this role. I foresee these Subcontrabass Saxophones becoming more popular, but not ever to the point of them being widely used. Even the traditional Contrabass Saxophone is rare enough.
Read more about the curious case of the Subcontrabass.
Albeniz’s Asturias for Subcontrabass Saxophone and Cello
A Bass Saxophone and a Contrabass Saxophone in octaves.