Diffusing bass frequencies would more or less turn modal ringing into reverb, which is not much of an improvement. However, there is one situation where a sort-of "diffusor" can help low frequencies. The image below is from the Diffusion video I linked above, and it shows how a stepped rear wall can help to break up reflections making them less coherent. Whenever I see someone with an odd shaped room having a closet (or vocal booth) in the rear, I often suggest orienting the room to put that odd wall in the rear.