I would use what the kit instructions recommend. I looked briefly at the long AVS thread, and I didn't see any mention of the type of stuffing. A photo showed fingered foam inside a cabinet.
These are ported cabinets, where bass performance is tuned by the port diameter and length. As long as the cabinets are lined with something, it shouldn't much matter what that material is. Many DIY newbies imagine different lining or stuffing materials can make a noticeable difference. Not really.
If you use denim, how would you attach it to the cabinet walls?
This is different in most sealed bass cabinets, where polyfill or denim stuffing (not lining) fills most of the cabinet's volume. The stuffing acts to make the cabinet volume seem larger, by as much as 20%. This can affect bass performance, usually in a small way.
In transmission line bass cabinets, the amount and location of stuffing actually is actually important. I've seen designs where an amount of polyfill stuffing is specified, and a different amount if glass fiber is used. But your speakers have a ported bass design.
