Polyfill or Fibreglass

Hello,

Based on my experience, these are my preferences:

Sealed Box:
I prefer fiberglass insulation as damping. It does very well at reducing standing waves as well as providing a good sound when using it to overstuff an enclosure, decreasing the alignment Q. I generally fill the cabinet, without "stuffing" (pressurizing) the insulation. I leave somewhere around 30-40% of air space by carefully wrapping the cabinet. In large cabinets (subwoofers), I find it easiest just to roll it into large circles, and stack the rolls as necessary based on the height of the cabinet. I generally use fiberglass only on large low frequency enclosures. If it's a two-way design, I prefer the generic poly stuffing.

Generic Poly-stuffing: good at reducing standing waves, but not good for more than a very loose fill. As such, I don't like using this for large woofers or subwoofers. For small sealed cone mid cabinets, I think it does a little better than the insulation, just based on practicallity of use. If you're trying to stuff a 4" or 6" piece of fiberglass insulation in a 2 liter mid compartment, it can get a little ridiculous...

Acousta-Stuff: I find this performs worse at reducing standing waves than well-teased generic poly-stuff, but slightly better for "stuffing" to adjust the alignment Q than generic poly stuff (pillow stuffing). I may use this in smaller to mid sized midrange cabinets. I personally find that it is "harder" and more reflective than generic poly-stuffing, and therefore can make resonance and standing wave issues more pronounced. With these complications, I generally avoid it... and it doesn't help that it is a lot more expensive than the generic stuff.

Real Sheeps Wool:
My cousin has a small herd of sheep and I get free access to all the clippings after he sheers them. I find it to be MUCH better than poly stuffing, and ever so slightly better than fiberglass. It does very well at reducing standing waves AND at altering enclosure size for reduced alignment Q. But, due to the fact that it's not easily accessible to the average Joe, I don't typically mention it.

Vented Box:
My preference is "egg-crate" style acoustic foam (I also find that mattress pad foam works quite well for this) along the walls. In addition, I prefer generic poly-stuffing, stretched to reduce density, and placed loosely in the large dead spaces. This combination helps a lot to reduce the standing waves and yet does not mess with the "enclosure size" acoustically (otherwise altering the alignment and tuning). I find that fiberglass insulation really messes with the tuning/box alignment.

General Thoughts:
IMO, any method described above is an "art." It takes lots of experience and messing around with each new box/speaker. I don't claim that my methods are best for everyone, but they are best for me... as of now. :)

Good luck experimenting! :)