Also, woofers get hot. The more stuffing, the hotter they will get and the more compression, I’m not entirely sure this cabinet ever had any stuffing at all, it looks way too clean. I would reach out to REL and ask if it used any first.
If you decide to add fill, Acousta-Stuff is a pretty good fill but for damping panels I much prefer the multi-layer Sonic Barrier to any thin membrane stuff. Comes with PSA already applied. I would start by playing music and feeling the cabinet for resonances. Apply the thickest possible Sonic Barrier there.
Then, if appropriate, lightly fill the cabinet. Make sure you do not insulate any hot areas such as internal heat sinks, metal surfaces, and leave room around the woofer for it to breathe. Woofers get hot when used, the better cooling, the less compression.
Best,
E