Below 500Hz, considerations should be placed on the thickness of the material you choose, this is very important. Personally, I would go for a 4-inch fiver glass insulation with 3-lb/cu ft density placed about three inches from the wall. This will hold a significant improvement between 125Hz - 4KHz, which will more than cover your needs. Remember not to over saturate your room with this stuff and strategically place it so covers the areas that need to be covered only (this depends on a number of factors such as room size, shape, materials already used in construction and your weakest points in the room acoustically i.e your windows and doors!). This in turn will allow your monitors to be more honest with you about your mixes! Good luck!