I'm currently listening to the Brainwavz HM5, another headphone in it's pricerange.
Some impressions vs the HP100, but i honestly have to say this feels like an unfair comparison as i wouldn't group the HM5 into direct rivalry but more as 1-2 classes lower.
Bass: The HM5 has no chance here, the HP100 goes deeper, has more subbass with more definition and lack of distortion, more impact/punch and overall 2-3dB more upper bass. The bass on the HP100 is the foundation of an extremly coherent presentation. The HM5 feels more seperated into bass,mids,treble. Drumkits sound alot better/natural on the HP100. Electronic music sounds alot better on the HP100.
Mids: Mids are a bit thinner and more distant on the HM5. The HP100 has more upper mids and more clarity in that region. HP100 sounds more lively with vocals and crunchy with guitars. Depending how sensitive you are this might be too much, but it's nowhere near grado level. For my tastes it's a great balance of a gradoish sound with more hifi-like attributes.
Treble: It's there on the HM5, not as forward as the HP100 and not as clearly defined, kinda unobstrusive. HP100 has a smoother treble presentation.
Soundstage: Less width but more depth on the HM5. What's interesting is that the soundstage seems to reveal more details. You will hear spacial cues/reverb that you don't make out on the HP100.
Other things (subjective): The HP100 has a smoother sound to it and is more _alot_ more involving. Comfort is better on the HP100, the HM5 clamps too strong like the HD600/650. In the end these two headphones are kinda similar but the HP100 is so much better at everything that i wouldn't really consider the HM5 if you can get the HP100 for the same or a little more.