It's come to my understanding that amps can still sound vastly different, even when all of them measure perfectly flat from 20-20k on a frequency response chart, and all have similar distortion characteristics. Don't ask me why, but there's other factors involved.
This isn't the first time I've heard someone say the O2's bass is rolled off or sounded too thin for them. Yet at the same time, the epitome of subjective reviewers at Headfonia loved the HE-400 with O2, despite disliking the O2's sound in general. I'm sure 90% of it boils down to personal preference, MLE giving his own perfect example. Whitecrow has also said he didn't think the Lyr did anything better on the HE-400 compared to a LDMKII, the likes of which I had and ever so slightly preferred the O2 over.
As for the HE-400 sounding better over time, 100% it's brain burn-in. It can be such a dramatically different signature than most other headphones, and if you'e coming off something with the slightest hint of brightness or thin-ness, the HE-400 can easily sound muffled upon first listen. It definitely did for me, and I was coming from an HD650! It only took me an hour or two to adjust, and the HE-400 quickly became my neutral again.
My sentiments exactly. You can't measure everything, and the components of subjective impact probably have equipment-level factors not captured by existing measurements, or there are external environmental / cognitive / physical factors (like someone living in a noisy city v. country, having a strong brand bias, or even slight hearing loss). That said, I would rather have a flawed standard measurement approach than none at all.
+1 on Brain Burn-In. The listening experience of the HE-400 was disappointing to me at first - I expected a more "V-Shaped" response, and these seemed anything but. I expected to be blown away, I wasn't. Then they became the most transparent and textured things I'd ever heard. I'd bet now that many, many other fine phones would sound frickin' terrible A/B to these, and to my ears.