Just another quick comparison I did over the weekend - I compared the HP100s to the A900x headphones.
First off, just comparing aesthetics, I do like the look of the A900x headphones more than the HP100s. That's not to say the HP100s are ugly, they aren't. I just wish they did away with the glossy ear cups. The A900xs have a nice look and feel to them. Build-quality could go either way. I think the A900x's are well built. They feel sturdy. The HP100s also feel well-built.
Now, comfort is where the two headphones are very different. I think the HP100s are very comfortable, and this comes from a guy who normally can't stand leather pads. Somehow the HP100s just disappear on my head and I forget they are there. To me, they are even more comfortable than my HD800s. Yep. The HD800s are comfortable, but after wearing them a while, I start feeling pressure from the headband and even the ear pads. The HP100s don't do this. Comparing the HP100s to the A900x's - the HP100s easily take the comfort crown. The A900x's use the ATH Wing system and this just doesn't work form me. It feels like the headphone is about to fall off at any time. I don't like it.
Sound-wise, I will just get right to it and say that I prefer the HP100's presentation over the A900x's presentation. The A900x has a warmer signature than the HP100s. I feel that the HP100s retrieve more detail and present it in a more neutral way. I love how the HP100s give me all of that detail in the highs and mids and still manage to produce such deep sub bass that doesn't overpower anything. The A900x's did impress me though, they are nicely detailed with full mids and nice bass. If I had never heard the HP100s, I might have just kept the A900x's and would have been happy. However, when comparing the two, I just couldn't shake the impression that the A900x's came off as a little boomy and less refined than the HP100s. I also liked the soundstage on the HP100s more than the A900x's. I am still impressed with the HP100s ability to produce a very nice soundstage with great imaging.
I am still very impressed with the HP100. It continues to perform above it's price range and outperform other closed headphones.