"Stuffing will raise the effective volume..."
Well, imo, yes and no. Stuffing will increase system damping, but the measurements I've seen, and the professional-level modeling programs I use, indicate that its effect on low frequency extension is insignificant. So we can't buy "free bass extension" by adding just the right amount of our favorite stuffing material.
Ime the slight softening of "impact" from adding stuffing to a subwoofer usually outweighs any benefits (assuming the system is well designed to begin with), BUT not necessarily...
For example, in a situation where room + sub = boomy, aggressive stuffing of the cabinet can be a net benefit.
If it's a ported box in our "room + sub = boomy" scenario, often we can make a greater net improvement by lowering the port tuning frequency, which may call for some ingenuity.
Duke