When I made my first instrument, a viola, I spent a good deal of time on line and on the phone with Mr. Zaret...very personable and persuasive man. I build the Zaret bar exactly as he told me. The result was that viola has a pretty small tone, nothing to be proud of at all, though it is a pleasant sound.
After building 14 more instruments (including only one bass, so far) I am convinced that the Zaret bar was simply a mistake. As I still have that viola (though it belongs to my son, now) I intend to open it up, remove the thick block of wood (Zaret bar) that it now has, and install a conventional bar (though probably still fairly high, and fairly narrow).
He has changed his design a number of times, and I still think it is some wrong-headed thinking.
It may help filter out some frequencies one doesn't like, but I do not think it will ever result in MORE sound.