I own the Soundmagic HP100 and tested the K550 today, although quite briefly. The Soundmagics fit better as they are smaller and seal better. I held the K550 for a proper fit. They both have better than average soundstage and imaging, both never sounding congested. They both aren't thick or warm sounding headphones but the Soundmagic is more neutral and the K550 more bright. The K550 has lots of detail but its treble is a bit too forward and can be too bright while the Soundmagic have detail but its treble is far smoother. The mids seemed similar with both being on the more detailed less thick less body side of things. The bass was where these are quite different. The K550 has bass thats fast and detailed but does what most do and gets weaker as it gets deeper. The Soundmagic has more body and manages to keep its weight down to the sub bass levels which makes them very impressive with various electronic genres or movies they can actually vibrate the cups with their massive 53mm drivers on the right track but don't get the impression that these are basshead cans, they're not. They really don't sound thick or bassy on tracks that have no bass. They are very smooth with a sense of effortlessness to them but some criticize them for a weak upper bass/ lower mid band. To me its not that noticeable and is perhaps due to an effort to avoid the all to common bass bleeding into mids issue that plagues many headphones. It lacks an area of bass that the Momentum has tons of but has the sub bass it does not.
Obviously, I favour the Soundmagic and wrote more about it because I know more about it. I just feel that this headphone deserves more props because its not only me but nearly everyone who owns one has had a great experience, and it seems many are missing out due to lack of visibility and availability. Rant over hope this helps others in the future.