Well, I was able to procure a brand new HE-400i from a HiFiman dealer here in Beijing. After pairing this with my Soundblaster Omni and Burson Soloist SL, I am quite frankly amazed at the improvement in both sound quality and comfort over Soundmagic HP100. Over the past few weeks I've tried some other headphones, and I feel the build quality of the 400i is much better than that of the Sennheiser 600/650. I tried the HE-400 and HE-500 but ultimately found them too heavy and cheap feeling for my preference. The HE-400i is a great improvement even over the 400i and alcantara pads, due to the pleather/velour combination pads.
In terms of sound quality, I find the bass punchier with more impact, and the midrange and lower treble more distinguishable than the HP100. After several hours of listening, I found that the HP100 w/ alcantara pads seemed to muffle certain instruments and percussion (easy to tell when listening for snare rimshots or cymbals). The 400i has none of these problems and I find it easy to separate the instruments in my listening... although not quite as well as I could with the Senn 650's. The soundstage seems a bit bigger than the HP100, but not remarkably so, and definitely not as large as the Sennheisers. Yet because instrument separation was so much better, I still felt the overall "stage" was improved.
I finished Homefront today, played some more Jazzpunk and completed the first chapter of CoD Blops II. I find the 400i to be more immersive due to the more even sound signature and bass/sub-bass extension. However, the HP100 was no slouch either and I still switch to them when I need a closed headphone if the wife is sleeping. Although the HE-400i offers a definite improvement over the HP100, I feel some of this is lost in gaming - the most marked differences occur when I listen to music.
Either way I am quite happy with both headphones. I have a Resonessence Concero coming in to replace my Omni for music, so I am hoping it will offer some aural improvements as well.
I'll post further impressions of the 400i in terms of gaming later