I'm currently working on a flame maple/spruce classical guitar using a shallow tornavoz. It has the effect of making the bass notes clearer and the treble sweeter by lowering the Helmholtz frequency of the soundbox.
The tornavoz was used by Torres on some of his most famous instruments of his first epoch. His were made out of brass and were deep.
In this case, I'm using a modern design. The ring is made of wood. This design is attributed to Austrian luthier Tobias Braun.