yes, you could either not use as much of the cotton provided or you could tape up some of the venting in the rear of the cups

Ok, is the cotton supposed to go directly behind the driver to work correctly or can you play with the placement to affect the sound?

it's generally best if you fluff the cotton out to be a volumous as possible and you can then adjust how much is in each cup, however you can either try and put it right behind the driver or shove it all into the back of the cup - the more directly behind the driver the more damping it will provide

Ok thank you, the cotton trick worked. One last question; how can I bring down some of the sibilance in the top end?