Ok. The M100 has first a superb bass response, I'm a bass head. Then the soundstage though not denon old series level, is pretty good for its price. The d600 just doesn't do it for me, for some reason TO ME, it sounds boxed in not compared to open cans but even when compared to say the m100. Possibly resonance? I'm not exactly sure but well, maybe it's the shape of my ear canal?
IMO, the d600 isn't worth it's MSRP to me, the M100 is a very strong contender if you are looking for big bass but not sacrificing sq, at this price range. However, I have to say I don't own these headphones, I only auditioned them, but apparently the d600s boxed in problem was confirmed when I tried the headphones twice and also checked with the salesperson that it is full burnt in. They are also very well powered with a RSA Darkstar and a Naim Dac.