Are the screws attaching the driver to the cabinet tight?
You say you love the sound of the KEF, so why try to change the sound? Stuffing any type of dampening material inside the loudspeaker will radically change the loudspeaker's sound. You'll be changing the internal volume which at the very least will change the bass response. I think I understand what you are trying to do with the lead shot, but why not just wrap it alternating layers of duct tape and bubble wrap. Either solution will only slightly mask the real issue inherent in the KEF's cabinet design. All box loudspeakers experience cabinet vibration and it's not necessarily a bad thing. If I were you, I would just continue to love the sound and stop worrying about the vibration.
You say you love the sound of the KEF, so why try to change the sound? Stuffing any type of dampening material inside the loudspeaker will radically change the loudspeaker's sound. You'll be changing the internal volume which at the very least will change the bass response. I think I understand what you are trying to do with the lead shot, but why not just wrap it alternating layers of duct tape and bubble wrap. Either solution will only slightly mask the real issue inherent in the KEF's cabinet design. All box loudspeakers experience cabinet vibration and it's not necessarily a bad thing. If I were you, I would just continue to love the sound and stop worrying about the vibration.