To my ears, the M100 seems to extend the same but they have so much better quality than the D600s... If the M100s had the level of comfort of the D600s, they would have been great!!!
Still, the D600s are Ok headphones.
Are they great sounding headphones (better than the D600s) at better prices? Yes... So if someone is looking for Denon because they plainly like Denon, I can see the D600s being a good option! Other than that...
Would like to try out some M100s, sadly they are not available in Sweden.