The most notable difference between the K550 and the Soundmagic HP100 is the bass region. The K550 has a softened attack and bassdrums sound "fluffy". There is less midbass and subbass with the K550. The HP100 has a more defined and punchy bass region. Also the bass, mids and highs are more detailed and resolving.
I still really like the K550, the bass has it's own character and is enjoyable for people that don't need their ears punched out by bass. It could even be called "tube"ish.