polyfill/fiberfill,fiberglass/mineral wool/rock wool can all do those things for you. it all depends on how much of it you use and how you use it. can even combine them in different amounts to achieve specific frequency absorption. polyfill is usually not a good absorber but it's great at slowing down reflections in the enclosure and makes the driver believe it's in a bigger enclosure. usually recommend to bring the mid to high frequencies out cause it has the biggest affect on those cause midrange and high frequencies are very short sound waves so stopping them is not so much a difficult task. most problem with high frequency roll off if enclosure reflections. frequency clash and resonate which can cause dips or peaks and of course cancellations at certain frequencies depending on the enclosure and driver used but, mostly what happens is if the enclosure resonates too much and back of the driver is not protected or has proper absorption, high frequencies tend to cancel out first and then down to the midrange as well depending on how bad the resonance is inside which will cause a shouty/boxy sound in the midrange.
fiberglass/mineral wool/rock wool are meant to absorb. thicker/ more dense it is, lower the frequency it can absorb. you can completely use thin layer of fiberglass to completely absorb high to midrange frequencies and eliminate complete resonance within the enclosure but too much will damping the driver too much and cause unnecessary major frequency roll-off depending on the enclosure and how thick/dense the material is. that's why it's good to combine if you have to. like use fiberglass for low-frequencies and use polyfill to help slow down the short-wave reflections to prevent it being completely absorb by the fiberglass and will help prevent from midrange to high frequency roll off but since this is headphones it's more complicated but the same fundamentals apply as for speaker boxes since headphones are nothing but little speakers but are design just a bit different since they are completely full-range so special attention and creativity needs to be applied.
look up speaker and room acoustics for more info on the materials. audiokarma has tons of data for absorption properties of the materials and will tell you the density needed, but off course you don't need as much since as it says since of course we're dealing with headphones instead, so gonna have to experiment a little, but in all the driver in your headphone can just be limited and is not capable of reproducing the higher frequencies as it's need to. who knows. hope this helps a little.
Edited by RexAeterna - 4/4/12 at 12:53pm