Hi there
The rubber surrounds do that, which is very annoying, but there are 2 ways to cure the problem.
Firstly you buy new drive units from B&O. easily fitted yourself, or they can do it for you.
Or you can refoam them yourself. The foams are available cheaply on eBay, and easily fitted with some latex adhesive. By far the worst bit is removing all the old foam. It goes like tar and is a messy stuff indeed. Other than that, the job is easy 
And yes, CX100s, are a bit bass-light, the CX50's I have are even moreso...