Once the regulation specs have been verified as correct, if you still have hammers that "bubble" on a soft blow try bending the damper lever springs so that it exerts less force against the lever. I know this sounds unrelated, but if the bubbling is eliminated when the damper pedal is depressed, then reducing the tension of the damper lever springs will do the trick. Our damper lever springs are pretty stiff, given the task, and the damping won't suffer.
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