just got these in. Had a listen, and was able to make some quick, early comparisons with the Hifiman HE-4xx:
1. Treble: the treble on these is still detailed and sweet, but definitely less extended-sounding and less pronounced compared to the Hifiman. I wouldn't say that there's a veil, but treble is more polite and subdued in relation to mids and bass. That's not to say that the Hifiman is piercing; So far I prefer it for treble detail, and it delivers without sizzle.
2. Midrange: Both have a solid sounding midrange, though the HD58x midrange sounds to me more pronounced in the mix, both upper and lower midrange. I don't hear a hump, but it can sound a bit thicker and maybe a tad woolier in the lower mids than the Hifiman. Again, both are pretty neutral sounding, and some may prefer what the Sennheiser does for slightly bringing male vocals more into the mix.
3. Bass: The HE4xx seems to have flatter bass and more extended sub bass, but can sound a little lean in the upper bass. With the treble, it's a 'faster', livelier sounding headphone than the HD58x. Again, they're both pretty neutral sounding headphones, so the differences are not so huge, but this one's still pretty apparent. The HD58x is a softer, more rounded sounding phone with a somewhat meatier upper bass boost. As such, the bass doesn't seem to seem as tight as the Hifiman.
4. Sensitivity: The HD58x is considerably more sensitive than the HE4xx. Quick A-B switching on an Onkyo DP-X1a (high output setting) reveals that the Hifiman requires quite a bit of volume boosting to achieve the same SPL. I think this shows that despite the low output impedance of the Onkyo, headphone sensitivity is much more noticeable and important with portable devices than is headphone impedance. Despite the high 150 ohm impedance of the HD58x, it was much easier to drive to higher listening levels.
5. Comfort: Yes, the HD58x were tight fitting out of the box, but after some stretching they fit very comfortably. The earphone oval shape, clamping force, and lightweight of the HD58x mean that it stays in place on the head much more securely than the HE4xx. The HE4xx is comfortable out of the box with nice thick pads, but it is heavy and can slide around more. Both are comfortable in a seated, immobile position, but the HD58x has a nice, form-fitted feel and lightweight that make it the more comfortable of the two for me.
6. I'm looking into balanced cables for the HE4xx to boost output; has anyone tried balanced with the HD58x? If so, does it make a noticeable difference in sound quality?
So far, I'm happy with both, but probably a bit happier with the HD58x, despite the sound profile of the HE4xx being suited more to my taste. It's still a great headphone for rock music, but the Sennheiser definitely has other strengths as well.