I love my M-100's but I find them slightly awkward when walking about/jogging with them for example. With the way they are closed, you can hear every step you take, but with a deep boom and amplified. It makes listening to music quite uncomfortable.
Must be the closed nature.
I used to notice the footstep "boom" also, but for some reason it's gone now. I don't know if I'm used to it or the pads broke in enough to alleviate the problem. I actually notice the boom much more with my Meelec A151 IEMs.