Got the Headsix from Tjumper78 today. I think it does what I wanted -- tighten up the bass from the headphone out jack on my Zune 80. But I'm still evaluating the purchase on a per-dollar basis. It was not a night-and-day experience, but that's likely a consequence of having to use the Zune's HO. Though it should be said that the Zune's HO is remarkably quiet. With volume on max and music paused, there is absolutely no noise on my IEMs with no amp in-line. With the Headsix turned up, there's just a whisper of hiss -- it's not noticeable during playback of anything but an older, hissier recording -- and there's an unfortunate crackling when I adjust the amp's volume, as well.
There seems a little bit more growl and bite to the bass when I listen to Radiohead (as I am right now: "Myxomatosis"), and there seems to be a little bit more depth and roundness and resonanance to the double bass on Miles Davis's Kind of Blue
I also seem to detect an improvement in midrange and treble. I can understand some Thom Yorke lyrics I'd never understood before (though I hadn't been using the new IEMs for that long before I got the Headsix, so this might not be fair), which argues for an improvement in the midrange. I don't know how else to speak about the midrange: it seems to be the part of the sonic spectrum that I hear least precisely.
The most marked improvement is actually on the high end, where I'm getting a markedly airer sound. Cymbals sound finer, snares more three-dimensional, and there's perhaps a slight improvement in separation that's most noticeable up top.
This has all been somewhat hard to evaluate, though, as like most people, I tend to interpret an increase in volume as an increase in sound quality. The Headsix is set for low gain, which effectively seems to turn it into an attentuator. If I have the Zune at 10 (of 20), 01:30 on the amp is still a good deal quieter than the unamped signal. Setting the Zune volume to 8 is a loud-comfortable listening level; if I leave it there and turn the amp to 03:00, I seem to get about the same volume as without the amp. My most recent tests have been with this configuration.
I'm no sure if that's the best way to listen, though. Most threads I've read suggest that when listening to amped phones through a lineout, you should crank the volume on your source to max, then use the amp to control the volume. I'm presuming that means that all amps are attenuators, as I've never had a source I could comfortably listen to a max volume. But I'm not sure why I'd want to crank any source's internal amp; wouldn't that just introduce distortion, which would then be amplified by the external amp? Any reasoned advice? Should I find the middling volume on the source that lets me use a middling volume on the amp? Or should I use a high source volume and a very low amp volume?
And while I'm at it, how the h--l can any amp take an imperfect source and make it sound clearer and more open? I think I'm experiencing that, but I'm worried that I'm fooling myself!