Insulation wouldn't really effect the high-frequency, though maybe they were just offering an opinion in general. Back when I was a kid, and had ordered a set of components to make a mono 030 into a stereo pair, while waiting for them to arrive I built an MDF copy of a 433X cabinet. I tried it without interior insulation first and it sounded kind of hollow. Much improvement with fiberglas batting added. The benefit of interior insulation or padding is best described
here: (monitors, delete the image if the size becomes a bother, I coudn't resize it and the link should work fine.)