The photo shows the unobtrusive modification.
Below is a short soundclip - a phrase of music before the Tornavoz and a phrase after.
The resonance of the bass has been lowered by 5 tones, from the open 5th string to the open 6th string, as you will hear.
Note - you won't hear any difference if you listen on a laptop or a PC with a small speaker - you need good speakers to hear the deep bass!
The music (listen here) moves from "before" to "after" without stopping, at the halfway point
The bass with the port fitted
(nestling between prime and alto)