Hmm. Its hard to say. I've only had them a little while and they haven't gotten a ton of head time. To my ears they are slightly warmer than the HD600 but it could be that the treble just doesn't have the same sparkle and quality. Overall I was quite impressed with them but in terms of the congestion & grain we mentioned earlier, they have a little more than the HD600. I really like the no-nonsense design but they don't feel all that secure on your head and I found myself longing for the more substantial build of the DT880 and HD600. The center image can sound a bit congested and generally they can sound a tad plasticy at times but only slightly. They are actually really impressive headphones considering how little attention they get. People who are used to 'phones like the DT880 or HD600 would feel right at home with them in terms of overall technical ability. Despite the niggles I've described there's really no obvious weaknesses that jump out at you. If for some reason all of my other headphones were gone and I had to live only with the GMP400 I wouldn't be missing out on much. I think they compete with DT880/HD600-level cans better than their lesser-known status suggests.
EDIT: The phrase that comes to mind when I try to describe the GMP400's treble is that it "breaks up" at times, but I'm not even sure if I'm using that term properly. I've heard it used before to describe treble that isn't particularly coherent compared to some of the better cans out there and that phrase tends to come to mind but its really hard to say for sure. Every time I put them on I'm pleasantly surprised. They have no obvious weaknesses, just flashes of slight flaws here and there. They are definitely an interesting headphone and I think I need more time with them before I'll feel entirely confident about their personality and technical ability.
Edited by devhen - 3/9/14 at 4:40am