There are two types of earphones that Sennheiser produce.
1) Traditional earphones which sit loosely in the ear (model numbers starting MX).
2) Ear-canal earphones which sit tightly in the ear (model numbers starting CX or IE).
The MX series sit loosely in the ear and do not rely on a really good fit to reporduce the sound.
The CX or IE series require a good fit within the ear canal to reproduce the sound. For this reason we supply different sized ear adaptors. These are the soft tips that fit on the earphones.
Please try out all sizes of ear adaptors to find the best fit for you.
If you do not achieve a good fit in your ear so that there is a seal all the way around the driver, then this can cause the earphones to sound thin or tinny, with no bass.