the way you will notice the perfect fit is that the bass will go heavier, because if the fit doesn't seal the ear canal perfectly, then the amount of the bass is lesser than the perfect seal (you can notice it when you using the foam tips, when you squeeze the foam tips and insert it to your ear canal, you will notice directly that there is no bass when the foam tips still on the process to fill your ear). For me I use the silicone tips and it provide me with a perfect seal (the shell of my SE215 is completely attach or having a full body contact with my ears). the advantage of the silicone is that it doesn't make your ear canal feels hot and it doesn't attract much earwax as the olives does, but the disadvantage is that the sound can be covered when you open your jaws (Example like yawning or eating), while the olives can make any shape of your ear canal, it will follow the shape of your ear canal even when you open your jaws. The disadvantage of the olives is that since it is a foam, it will make your ear canal hot (for like 1-2 hours of continuous using) and you will need to squeeze it before you insert it to your ear and it will take time for the foam to fill up and cover your ear canal (for maintaining perfect seal), the most important part is that the lifetime is shorter than the silicone tips (my silicone tips is like permanent lifetime compare the limited lifetime of my olives). So its still depend's on your ears though (mainly the foam is used when you want to perform on the stage or do some extreme audio mixing). Hope this help, Thanks.