Ear sleeves play an important role in hi-fidelity sound reproduction of IEMs. If the coupling of IEMs to human ears is not air-tight, various non-linear distortion inducing problems occur; loss of bass is merely a beginning. This is why IEM manufacturers include A TON of ear sleeves in their product packages so that customers can get the tightest sealing possible with their IEMs.
Of course, different sleeves demonstrate different quarter-wavelength resonant characteristics, due to the size of the bore being different. Considering the above, one can actually improve or degrade the sound quality of IEMs by simply changing ear sleeves. A rule of thumb: decreasing the bore length & increasing the bore diameter shifts resonant frequencies to higher frequency range, and vice versa.
Here's Etymotic Research's MC5 with different types of ear sleeves:
THINGS TO CONSIDER:
1. Ear Sleeves of Shure & Etymotic Research are compatible to each other; not sure about the ones made by Westone. Somebody get me some Westone sleeves, then I will include them in this analysis as well.
2. Silicone sleeves offer better bass extension, due to the silicone's water resistant property.
3. Shorter the bore length, the more even high frequency response for MC5.
4. Wider the bore diameter, the better treble extension for MC5.
Personally, I would like to choose gray sleeves among silicones, and shure olives among foams. Of course, all these can be achieved, only with proper insertion depth.