[blueangel2323]

The bass of the HP100 has a lot more energy and punch compared to the K550, or at least it's much easier to find given the finicky placement and weak clamp of the K550. The general sound of these two headphones feels similar in the detail retrieval and and isolation department. If you read my review of the K550 you'll know that I didn't quite get on with the fit or the sound, in fact these two aspects seemed intrinsically linked. My main problem with the K550 was a lack of bass presence and harsh high frequencies when trying to find that presence. The isolation was good, but only if you physically held them in place - it had a weak clamping force which ruined the isolation and the bass along with it. This gave the sound a claustrophobia that was chaotic and made compensation frustrating to impossible.
All of these issues are nearly non-existant with the HP100. The bass of the HP100 feels much stronger at the best of times, it's deeper, kicks harder and it doesn't effect the mid tones at all. Highs sparkle well too but don't have nearly the harshness of the K550 I think the HP100 is actually a much better fit for reference sound than the K550.

 
Source: http://noblehifi.blogspot.ca/2012/10/soundmagic-hp100-review.html#!/2012/10/soundmagic-hp100-review.html

 

[Grey Owl]

Thank you, blueangel2323. It was really helpful.

 

[Makiah S]

I got the K550, the fit was perfect and it's everything I can ask for in a closed can! 

 

[lumberjake]

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.