Pros: Very comfortable (thanks to suspended earcups), accurate sound, robust construction
Cons: Not as good looking as a Sennheiser
The GMP 400 is handbuilt in Germany and comes with a 3 year warranty. The GMP 400 is the open back cousin of the GMP 450, which has a closed back design.
The GMP 400's sound is very nicely balanced. Enough warmth to make them an enjoyable listen, with an incisive precision that allows you to really hear all the hidden detail in a good quality recording. I feel the balance is just right.
These headphones are like a mixture between all the things that are good about the Sennheiser HD650, the Grado SR325is and the Beyerdynamic DT880. Not in your face like a Grado SR325is, not overly relaxed like a Sennheiser HD650, and not overly analytical like an AKG K701/K702. More like somewhere between the Grado and Beyerdynamic sound signatures, which is a good thing!
The earcups are mounted on what German Maestro call a cardomatic suspension system. This results in a feather soft grip around the ears that readjusts as you move, which stays surprisingly secure on the head. A really innovative design.
Very well balanced sound, a high level of build quality and extremely comfortable fit make the GMP 400 a serious contender. These headphones should definitely be added to the shortlist and demo'ed.