If they are molded plastic then I would fill them with something dense like insulation or mineral wool to help absorb low frequencies.
Hollow diffusers are a strange animal. It is my belief that they reflect (or rather, refract) high frequencies, they exaggerate their own resonant frequency, and they are too small and lightweight to affect low frequencies. I would attempt to muffle them (similar to muffling a drum head) using duck tape or something similar on the inside surface. Then I would add porous density to the remaining air space inside.
They are used to reduce direct reflections which contribute to blurring, doubling, or comb filtering. Generally the rear wall of a control room is the best candidate to start with, although any wall or ceiling can benefit from diffusion.