Before I got the GMP 400 I found a few reviews online (not on head-fi) and I'm really surprised that none of them mentioned the bass. These are commonly compared the DT880 and HD600/HD650 and I'm surprised that I never heard it mentioned that they have more bass than either of those cans do. They also have more bass than the K712. It isn't too much bass but I consider the bass on those 3 'phones to be quite neutral so in comparison the GMP 400's bass definitely sounds north of neutral. I actually expected them to be a bit bass-light based on the descriptions I've read and the fact that there is virtually no clamping force and the cups are attached to the headband via elastic strings. Yet there's a remarkable amount of bass impact! If you want something similar to an HD600 or HD650 but you need more bass, the GMP 400 should be right up your alley.
Nice to hear since I just ordered a GMP400. Hoping this can will help curb my chronic obsession about comfort an attribute which, for me, can be elusive.