I'm not the best at describing sound so bear with me. The big difference is the bass. I like the bass of the HE-400s but it isn't as voluptuous as the Pro 900s. This seems to lend itself well in making the Ultrasones come off as the thicker sounding phone. It's got more substance in the low end but the character is less defined (though not to discredit the Ultrasones).
The HiFiMAN mids are stronger sounding, almost to an odd degree. It was actually a shock because vocals sound so much different. It's a bit more articulated. As someone that's spent a lot of time with the Pro 900s, it was quite an experience to try a headphone with slightly less recessed mids. These still don't sound flat but I think they use it to their advantage; flat sounds boring, in my experience. A slight smile in the response is more satisfying.
The highs are a wash, in my opinion. My hearing isn't the greatest (one too many rock shows and two too few ear plugs will do that to you) so I have a difficult time picking apart subtleties in this area. The HE-400's highs seem to have a bit more air underneath them. In both cases, I find them to be pleasant enough. I've got nothing bad to say about either set. I've heard less-than-flattering things about both headphones' high end but I just don't notice it. I'm happy with the amount of definition I get from either.
Aside from all of the frequency stuff that defines the signature, the biggest difference is the spaciousness of the sound. The HE-400s seem to place sounds all over the sound field. The Pro 900s never came across as being narrow or confined in any way but the HE-400s expanded on that broad stage, giving an extra sense of depth, almost at the expensive of overall tonal thickness. That's the vibe I get, at least.
There's also the comfort issue to consider. The HiFiMANs are quite a bit heavier and the cups are slightly bigger. They are both comfortable enough but the edge goes to the Ultrasones. I wouldn't turn down the HE-400s just because of their bulk and weight.
In the end, I'd say that going from Pro 900 to HE-400 is more of a lateral move. They have different sounds but one isn't obviously better than the other. They're different experiences and I'm happy with the money spent on both of them.
I'll see how my opinions on the matter change once I spend some time with both pairs and an amp. And, to be fair, I didn't spend a ton of time with the Pro 900s before putting the HE-400s back on and giving these impressions.
Maybe bareby can chime in on the issue. I've seen him around the HE-400 thread and here he is. ;)
Edited by LarryKingsShoes - 2/22/13 at 1:41pm