acoustical stuffing / sound dampening subwoofer cabinet REL BRITANNIA B1 subwoofer

"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

As auxinput states, internal bracing is the best and first way to combat enclosure resonance. Braces stiffen the enclosure walls, raising their resonant frequency above the frequencies the sub is asked to produce. Internal stuffing "tricks" the box into thinking it’s internal volume is greater than it actually is (by slowing down the sound), and needs to be matched to the driver’s needs in relation to that volume. Both too much or too little for any given application are possible.

Sonic Barrier is a fair enclosure wall damper, but a superior product to achieve that end is NoRez, sold on the GR Research website.