Just curious, all the cutaways only show aluminum.
I actually think polyfill is bad, but lining walls is good.
I think polyfill generally just make it sound bad in all the octaves but the lowest it seems. It might compensate for something else, but it isn't the right answer.
To me I see no limit in dampening the walls. And to a large extent using certain types of foam and things on them to reduce reflections. You need pressure, not reflections, inside a cab.
For me it's useful for tuning the woofer...
Damping the walls needs balance too though, if you add mass without stiffness you lower the first resonant frequency of the cab. If you're starting with something very stiff like aluminum though it might not be an issue. I'd guess magico uses a layer that forms a cld like dynamat on a car door but idk... Aluminum is a bit over the top for a speaker cab imo.
Just a correction: YG uses no filler/stuffing material, as they have their own resonance-damping technology.
http://www.yg-acoustics.com/userfiles/files/PDF/Technologies/YG_FocusedElimination_Oct13.pdf