Well, I spent a late night listening last night - was up well past my normal bedtime
The HE-6 are an interesting headphone, as I've said. There are some ways in which they are spectacular. And there are some ways in which I wish they were different.
All my comments refer to the HE-6 via the Leben CS300X, which drives them effortlessly - and that's no easy task! I hauled my high end digital rig - AVA tube/hybrid DAC with Assemblage jitter filter and Wadia i170 up to the Leben.
The HE-6 have an astoundingly open and transparent sound. In the midrange, the sound is so open and transparent that it's a little startling at first. VERY impressive They are as grainless and pure sounding as I have ever heard a headphone sound. And they absolutely excel at detail retrieval - it is possible to hear micro-details in the music via the HE-6 that are not as easily heard via even my other high-end cans. Some of this is due, I think, to what is a slightly elevated treble response. The HE-6 have such a pure, grainless, and effortless treble that the fact that there is a lot of treble energy isn't as bothersome as it would be otherwise, and it at least seems to me not to be very peaky, but rather a broadly elevated treble. I think, though, that there is more treble energy than is really natural. Sometimes this was thrilling, other times I found myself wishing that the response was a little warmer, and, IMO, truer to real life in that regard. It's a paradox, because in some respects the HE-6 sound more natural than anything I have heard (due I believe to their astonishing transparency and I think dynamics/transient response), but their spectral response is upward-tilting, and to a slightly greater degree than I think is ideal, within the context of an ultra-high-end headphone. While the Leben is a tube amp, it's not really lush or tubey sounding in and of itself, being really very neutral. But my digital front end is if anything just slightly warm. So it's not the case that my system was "cooling off" the HE-6.
The HE-6 have VERY deep bass, and the bass pitch and definition is truly excellent, but at lower listening levels, the bass is definitely in the camp of slightly overdamped. I think it probably extends truly to below 20 Hz, but I wished for a little more bass weight at the levels I typically listen at (80dBA peaks). Turning the volume up to get more bass (listening louder than I typically do) DID in fact seem to bring out the bass, to the point where it was plenty satisfying, and I didn't feel that it was overdamped. And I still was not bothered by the treble. However, I don't like to listen that loud (peaks about 90dBA). But for people who do, and have the amp to get it done, the HE-6 sound even better at louder levels. At my typical listening levels, I preferred the Audeze LCD-2's tonal balance over the HE-6, and in general, I enjoyed listening to the LCD-2 more in some ways when comparing them directly.
But then I would put the HE-6 on, and there was all this glorious detail and transparency, and the sound was thrilling. In spite of their spectral balance, the HE-6 never seem bright in absolute terms, and I was never bothered by excessive sibilance. Again, I chalk this up to the purity and the smoothness of the treble overall. I'd love to have them retain their championship-level performance in that regard, and still have just a touch more bass weight, and just a touch of additional warmth. While I think the HE-6 are very impressive, and enough so that I want to own a pair, I am certain that I will spend more time listening to the LCD-2, as the spectral response suites me better. But I am equally certain that there will be lots of people who will prefer the spectral response of the HE-6, and for those people, the HE-6 will truly be the holy grail of headphones.
Oh, and then there is the soundstaging. Again, thrilling is what kept coming to mind. The soundstage is huge, and exceptionally well defined, but not huge in a way that seems artificially in any way. It's really impressive in it's imaging capabilities.
If you can afford the HE-6, I would rush to hear them any way you can IF:
1. You either found the LCD-2 to be too soft in the treble, feel the Senn HD800 has about the right basic tonal balance but wish is was smoother and had more transparency and less grain, or feel the Beyer T1 needs more treble extension and more transparency
2. You have a high-quality speaker amp you can dedicate for use with the HE-6, or a headphone amp with much, much more power than is typical
The HE-6 is the detail and transparency king. The view into the music they provide is absolutely thrilling. But there are some considerations in terms of partnering equipment and in terms of their tonal balance that need to be made with some care. For me, their performance overall is so enthralling that I am buying a pair. But given the amping requirements and the kind of sound they offer, some careful consideration is required, as it always should be when spending considerable money is part of the equation.
As such, I'd really like to thank Fang for putting this loaner program together. It's an invaluable service.