When you (scallop) or remove wood from any brace you weaken the brace. This allows that particular area of the top to be more flexible which allows it to move a greater distance, at a slower frequency and will "usually" change the bass response. Adding mass to a brace "can" have the same type of effect which sounds counter intuitive. The brace can be heavier but more flexible, based on its design which will allow the top to move a greater distance albeit at a slower speed, which will change bass response too.
If you are doubtful stick a C-clamp through the sound hole and clamp it to your bridge and listen to how bass heavy the sound of the top gets. The C-clamp didn't change the stiffness of the brace but only added mass to that area of the top. There is a difference between adjusting stiffness and mass and knowing how to manipulate both is one of the many ways to tune a top.
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