A little while back I purchased a set of Apple Dual Driver in ear monitors (ADDIEM) from a fellow head-fier. I had a terrible time trying to get a proper seal with the standard tips. I read a suggestion to try the Sony Hybrids and I snagged a set at Best Buy. I put a set on the ADDIEMS and achieved a good enough seal for front of the plane air travel noise isolation. I began experimenting with some of my other IEMS (I am a sucker for cheap IEMS) The hybrids improved the fit and sound of the Zune Premium, the Meelec M9 and the Maximo 490. I looked on ebay for a seller of pseudo hybrids and someone here had recommended the seller "lostearbuds." He offers 9 pairs of any size combo for $6.99 shipped. I ordered some and they arrived in 2 days. I now have hybrids on just about every IEM I own. I recently acquired a set of Monster Beats Tour In ear and found it difficult to achieve a seal. In reading reviews around the web almost all reviewers had this problem. A set of hybrids fixed that problem. I have no idea why this seems to work so well as there isn't an obvious (to me anyway) difference in the properties of these tips. They just fit. I highly recommend spend $7 and trying these out on your IEMs especially if you have comfort or fit issues.
Here is a link to the eBay seller mentioned above: