A diffuse sound field requires that more reflections take place in order to spread the sound out evenly, and the result of these many reflections is a faster decay rate than a non-diffuse sound field.
In a domestic size room, it is very difficult to achieve a diffuse sound field at anything other than the high frequency range. Even though we might employ special diffusing surfaces, the sound is far from truly diffuse in the lower transitional range. As such, we should pay attention to the time it takes for sound of different frequency ranges to decay. It is of desirable for that time to be pretty much the same across the audible range, so that some frequencies don't hang around for longer than others. Given the lack of diffusion in the lower transitional range, we often see an uneven decay rate. The image below shoes just this; taken in a fairly bare room with nothing other than carpet and a chair, the rise in decay time around 1.7KHz shows a lack of diffusion.
If we have already employed diffusor devices, then the remaining option is simply to increase absorption in this range to smooth the decay time.
The decay time for a domestic listening room should be about 300ms across the range for a controlled fast decay room, to 500ms across the range for a live spacious room.