From what I understand( and someone please correct me if I'm wrong) the box size can vary about 10% before parameters change , so the stuffing should not drastically change the volume the speaker "sees".When stuffing the box , don't pack the wool or whatever you are using too tightly and make sure it is away from the woofer. Also if you are using a ported design keep it away from the port as well. I believe the object of stuffing the box is to eliminate standing waves.