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Sennheiser CXC 700 In-ear Active Noise-Cancelling Headphone Review$229.95
In the short term, several around the office reported that the initial fit of the Sennheiser CXC 700s is about as comfortable as in-ears tend to get. Overall, though, the fit depends on how well the included tips go into the ear canal. Because there are only 3 sizes, some may find that they can't get an ideal fit, but at least it's better than just having one size.
Over time, the fit doesn't really change, and the only thing that will change that is tugging from the massive remote, which we recommend you clip to your shirt. Barring any tugging on your in-ears, the fit shouldn't change.
Aside from choosing which size tip you use, there really isn't much you can do to customize your Sennheiser CXC 700s. It's not necessarily a bad thing, however, as these things are little: you probably won't have any other need to mess around with them.