I totally removed grills from my Koss UR40 (same drivers as KSC75), now the only thing between the driver and my ears is thin felt. And sound has changed significally, it's finally opened up! In stock they really bored me with sibilance and harsh treble, now this problem is partially solved. But i don't recommend to remove the foam from back side - it makes the sound too tinny.
Those drivers can be damaged easily. You may want to do something like this to protect them: