Summary of my findings placed in one post together:
This weekend I spent a lot of time listening to the HE-6 with ZDT amp via the PS Audio Perfectwave DAC (transport being Macbook, Samsung DVD and Airport Express 802.11n). I compared the HE-6 to my recabled HD800 and my Sennheiser HE-60 on the balanced WES.
POWER: The ZDT has less power than the SAC KH1000 amp when driving these phones, but a little more than the Nuforce HDP. It sounded the same whether I used the 8-watt speaker terminals or the 4-pin XLR, which is not surprising since they come off the same transformer tap.
Normal listening was conducted with the ZDT volume knob at 2-3 o'clock, although I could go louder to 5 o'clock (max) without distortion. This was fairly loud but not up to uncomfortable levels like with the SAC amp, although the ZDT at max volume is already higher than I normally listen. I'm thinking you might need to double the power of the ZDT in order to replicate the impact and volume of live concerts, which the SAC amp does seem to offer with the HE-6. However, I don't very often listen at those levels, and I think that the ZDT's refinement and balance outweigh the additional volume I can get from the SAC amp.
In comparison, the Nuforce HDP was about as loud at 3 o'clock as the ZDT was, but I could not get the HDP to play any louder than that without introducing distortion. So, I'm guessing that the ZDT amp had about 3dB more headroom than the HDP. In contrast, the volume knob on the WES with HE-60 was set for 9:30 - 10 o'clock for the same volume levels as the HE-6 with ZDT or HDP at 3 o'clock. The HE-60 could play loud enough on the WES to be uncomfortable before I'd reach 12 o'clock. I'm guessing the SAC amp with HE-6 can match those volume levels, but it's in a different rig so I didn't compare them. The ZDT only needed to go to 10-11 o'clock with the HD800, and 9-10 o'clock with the LA7000, to match the HE-6 at 3 o'clock.
The 5-watt Travagans Red with OPA627 opamps starts to distort at 2 o'clock, at a volume level that is slightly louder than the ZDT amp, which is surprising. However, it didn't sound nearly as good as the ZDT. I suspect the SS output maybe has a little extra current reserves than the transformer coupled tube amp at these impedances. I have an 8-watt Qinpu tube hybrid that I still need to try out, and a Nuforce Icon that is rated 12 watts into 4 ohms I can try.
SOUND: The HE-6 with a world class amp play on a world class level to me. In fact, at louder volumes the HE-6 remain better composed and less fatiguing than the HE-60 or HD800, similar to my O2 Mk1, Jade, LA7000, K1000 or HE-5 LE. The problem is that my best sounding amp is not my most powerful amp, and these need a lot of power to really shine. But we knew that going into this.
The HE-6/ZDT soundstage is a little smaller than the HE-60/WES, which is a little smaller than the HD800/ZDT's. The HE-6 was focused and precise like the HE-5 LE, yet more open and spacious than those with live recordings. Imaging is fairly precise and not too forward or too distant, but the stage is not as deep as the O2 Mk1 nor as wide and voluminous as the HD800 or HE-60. The HE-6 is not quite as forward sounding as my HF-2 (bowls), nor my old RS-1 with flats as I recall. The transparency of the HD800 and HE-60 is still a little better, but the HE-6 offered a nice and more intimate view of the performance (e.g. passes the squeeze the singer's butt test). They still sound a little more open/transparent than my LA7000 or HE-5 LE, and the HF-2 or HD600 cannot keep up nearly as well. The original HE-5 sounded open and airy, but imaging was imprecise and blurred vs these others, and you start to get the sense that the bass and highs are slightly tipped up above the mids. The HE-5 LE and HE-6 improve on this.
The HE-6 bass was stronger than that of the HD800 or HE-60, which are not lacking with these amps. They all had good bass detail and speed, but I preferred the HE-6 bass impact over the other two. In comparing the HE-6 bass to the O2 Mk1, I'd say the O2 almost sound a little bloated and not as tight in comparison, although the O2 impact is just right for older classic rock. Spritzer will tell you this is the fault of the WES amp. The HE-6 bass quantity vs the rest of the frequencies comes close to my LA7000, beating out the HE-5 LE by a small margin. However, the ZDT cannot drive the HE-6 to hit as hard as the more efficient LA7000 because the LA7000 can play significantly louder with less power.
The HE-6 mids are rich and vibrant, and as enjoyable as the others, with a nice sparkle and detail to the treble. It was only that final degree of air, ambience and out-of-head soundstage that was missing with the HE-6 in comparison to the HD-60 and HD800. But I have not heard any other headphone except the K1000 or Sigma/404 that could match their sense of openness either, and the K1000 didn't have very good bass while the Sigma/404 didn't have very good air and detail.
Basically I would say that the HE-6 are a step up in the bass and treble over the HE-5 LE, with similarly excellent mids. The original HE-5 have good bass impact in comparison, but it's highs are set a little above the level of the mids, and the original HE-5 mids don't have the presence and weight that I find with the HE-5 LE and HE-6. While I preferred the original HE-5 above my re-cabled RS-1, and preferred the HE-5 LE over the original, I think the HE-6 does the best of everything that the original HE-5 and the LE did, but all in one package.
I'm not going to rank these headphones my other flagship phones, but I would say that they all belong in the top tier of headphones. When paired with the right amp any of them can shine (or fail with the wrong amp). If the HE-6 were mine, I would probably continue my search for the best amp to match them, and I'm thinking we need someone with a balanced Beta 22 to try these out since that was the best amp I've heard with the K1000. I think that the EHHA was also more powerful than my ZDT, based on trying one for a couple of weeks with my K1000 before. A nicer speaker amp is probably a good idea as well, and I'll see if I can get to the speaker terminals on my 55 watt Yamaha receiver.
7/13/10 - Final Impressions:
As I promised above, I had try the HE-6 with a powerful speaker amp. HOLY COW! I've been listening to the HE-6 plugged into the back of my Yamaha HTR5560 home theater receiver for an hour now, set for 2 channel stereo and flat EQ, and the sound is very good - dare I say fantastic!
I'm using a DVD player for CD's via Coax digital, and an Apple TV with some lossless and 24/96 music, feeding the Yamaha which is driving the HE-6 and my Polk SDA CRS speakers (aka Stereo Dimensional Array Compact Reference System). I'd have to say this combination beats using ALL the other amps in dynamics and impact, while still offering very good detail and ambience. The ZDT is a little more refined sounding, but a bit softer sounding on the transients and noticeably less powerful. If the ZDT was as powerful as the Yamaha I'd be in heaven, but as it is this combo still sounds quite good.
Out of curiosity I compared the HE-6 to my reference speaker system. These Polk SDA CRS were the speakers that replaced my Quad electrostatics back in 1986, because I thought they were a little better sounding and easier to drive (before Polk went mass market). Going back and forth between the HE-6 and my Polk SDA CRS speakers (about 12 feet away and 6 feet apart) the timbre and tone is very similar, but the HE-6 seem to play both higher and deeper (and I've measured the Polk to be about -3dB down at 32Hz and 18Khz before). As I switch between them with the volume set at -30 dB, I find the HE-6 also play a little louder than the Polk at the same volume knob setting, unless I cut my distance to the speakers to about 5-6 feet. Switching my Velodyne subwoofer on and off makes little difference in the quality of the music with the HE-6, while it makes a bigger improvement with my speakers. I put on a movie with the subwoofer on and increased the sub volume, and it did improve the immersion in the experience through the HE-6; but I still prefer speakers for movies (unless I get the Smyth SVS for 5.1 headphone listening).
Now, there's a caveat. The Polk speakers have noticeably better soundstage size, while the HE-6 sound more forward and intimate in comparison, which is expected when comparing headphones to speakers, especially to the SDA CRS. The Polk soundstage, because of the "SDA" configuration, stretches from wall to wall and anywhere from the plane of the speakers to many feet back and forward. Often with the Polk I even get a surround sound effect (must be using the SDA interconnect cable), and for a long time I didn't use rear speakers when listening to movies with them. The HE-6 keeps the instrument placement of the speakers, but compresses it down to a smaller stage surrounding me from all directions, with some of the stage inside my head (like with other headphones). With the HE-6 I am sitting among the instruments and musicians, while speakers offer a stage in front of me. No surprise there.
Anyway, back to headphones - it seems to me that there really is something to driving these with a powerful speaker amplifier. Since many people love driving the K1000 with a Firstwatt F1 and RWA 30.2, I suspect that those amps might be just the ticket for the HE-6. As for me, I'd be happy just listening through my Yamaha for a while, and with the long pigtail speaker cable I can still sit in my favorite listening chair (which puts my head about 11 feet away from the front of the receiver).
AS A FINAL NOTE: Before I resorted to the 55 wpc Yamaha Receiver, I also tried the HiFiMan EF5 which has a slightly more laid back sound than the SAC amp, and plays a little less loudly with the HE-6. I'd say the EF5 can reach volume levels between that of the ZDT and those of the SAC amp, and it drives them a little louder than the HDP can. It's not a bad match for the HE-6 at all, but with the slightly laid back sound it may seem a little less detailed with the EF5.
I also tried the Qinpu A-3 8-watt per channel tube hybrid speaker amp with the HE-6 and it drives them at least as well as the SAC amp, in the brief time I spent with it, and I think it did a better job than the EF5 by a small degree.
PS: I have the stock RCA clear-top back in the EF5, and a set of GE 5670 tubes in the A-3 amp. Both of these are a tube hybrid with SS output stages.
[EDIT 8/19/10 - fix a typo where I wrote HE-6 where it should have been HE-60]
Edited by HeadphoneAddict - 8/19/10 at 10:23pm