I received a used pair yesterday after my curiosity got the better of me. I'm definitely a fan of the fast ortho sound and I'm also a little addicted to Hifiman, so take that for what it's worth.
I gotta say, these are incredible. With stock pads, the bass is not as bad as I would have guessed based on impressions. No oomph down way low but bass kicks have plenty of body. I put on the Hifiman pleather pads and almost immediately removed them for the wonky/hollow sound (as a side note, I still don't understand this. it seems every Hifiman headphone plays differently with the pleather pads. it makes some cans sound terrible and others decent, whatever). I then switched to the Focus Pads and found a very enjoyable listen. The Focus Pads give them more bass presence than the HE-4 or HE-6. No joke. Not in a bloated way either. The mids are right where I like them, maybe slightly shadowed by the bass, but the treble is spot on. My sibilance tracks were barely sibilant, which I can't say for the HE-4 and HE-6. This is the most fun I've had listening to drums anywhere around this price point ($225 used). Very very engaging. Very very comfortable. Very very efficient (happily jamming out of my phone and iPod). The HE-4 and HE-6 have the edge in terms of staging, separation, and resolution, but it's hardly a landslide. Really.
The HD600 vs. HE-400S comparison is more justified than I would have guessed. The warmth in their signatures is very similar. And the pricepoint is very similar. The HE-400S is not as balanced in the midrange, creating a slightly unnatural timbre compared to the HD600. The HD600 also has better treble extension and presence (coming from someone who doesn't mind the upper mid/lower treble on the HD600). Staging/space is better on the HE-400S though. The 400S also has better bass texture and speed but I think I'd prefer the HD600 bass for a more relaxing/musical listen. But, overall, the HD600 simply sounds better with upstream gear. Out of my iPod, the 400S is probably the best headphone I've heard. From Valhalla2, the HD600 has much better resolution/detail and the HD600, in general, has a better tonal balance.
Compared to the Oppo PM-2 I found the 400S to be miles and miles and miles and miles ahead. At half the price. WTF. The PM-2 is probably the most comfortable way to put me to sleep.