Thanks for the response, particularly as my post verged on incomprehensible. What can I say? It was late, and I'd had a few IPAs.
I won't quote the whole thing, since I hope this will arrive as the next post.
Last night, I would not have understood your post at all. As I mentioned, my Zune is simply silent through the HO. There is no noise floor hiss whatsoever at any volume, with or without source, with or without power, even through quality IEMs (super.fi 5 pros). I mean that quite literally: no hiss audible to my ears. Though, admittedly, I am 37.
And my Headsix has a noise floor that is inaudible until the amp is turned up to 75%.
Today, however, I received a cMoy BBv2.00 rolled to an AD823 opamp that I bought used, out of mere curiosity. Powered on, but at minimum volume -- with the Zune maxed, you can just barely hear the loudest of music passages with the amp at this level -- the noise floor is quite significant, and it only increases with volume on the amp.
I just did a little experiment. I turned the Zune down to 50% and listened through the cMoy, which I turned up to achieve similar volume to that provided with the Zune at 20 , and got more noise. I then turned the amp down a bunch and increased the Zune's volume. As you would predict, I got less noise (though there was still too much, this being a cMoy!).
Thus far, what you say makes sense to me and is reproducable. However, when you say that HO is inferior because there are two amps in line, I no longer get it. I mean, in principle, I do: clearly, you'd rather have a good amp working fairly hard on a clean signal than working less hard on a signal that's been amped at inferior quality. This is what I was trying to get at in my previous post, when wrote that I though a line out had to be pre-amp.
But it's implicit in what you're saying, as well as other things I've read, that the iPod's LO is, in fact amped. A player couldn't put out an unamped signal that's stronger/louder than the amped signal. Everybody says, "in absence of LO, max your volume to approximate LO," right? If there's an increase in quality from the LO (and I'd hardly question that there is!), and the signal is, in fact, highly amplified by the player, then the noise must come from the HO jack itself, or the circuitry between the amplifier and the HO jack. I suspect that the quality improvement comes from a cleaner/more direct signal pathway or a less lossy connection than provided by the iPod's HO jack.
Since my Zune offers such low noise, I find myself believing that an LO might be superfluous. Listening to the Zune unamped then with each of my two amps is enlightening. Unamped, I get a seriously clean, nicely detailed sound that's somewhat breathless at the low end and lacking in ultimate resolution (though still a healthy improvement on my wife's Nano 8gb) .
With the cMoy, I get much fuller bass (particularly with BB engaged), but the bass is so bloaty and the overall sound so hamstrung by noise, that I'm better off without it, unless maybe I'm on a plane or bus. Although it is kinda fun, too. Right now, I'm listening to Radiohead's "The National Anthem," with Thom Yorke playing one of the coolest basslines ever, and it's just so over the top!
With the Headsix, I don't get near the improvement in bass offered by the cMoy, but the bass is significantly punchier and "growlier" than unamped. Kick drums in particular are more resonant and seem to decay at a natural speed. High-end detail, particularly cymbals, is improved, as well. Timbres and tones are way truer than with the cMoy. When I turn the Headsix way up, though (and that's only safe if I have the Zune at 30-40% volume) I do get a bit of noise added from the external amp.
As my understanding exists now, I'm feeling a lot better about choosing the Zune. The biggest reason I never strongly considered an iPod was sound quality -- remember that I had never even heard of portable headphone amps at the time -- and I thought the lack of LO might have bottlenecked me. But empirically, it doesn't seem to be the case.
Actually sound quality was the second biggest reason, If it were the biggest, I might have gone with a Cowon or Creative. The biggest was the customization offered at zuneoriginals.net. I'm a sucker.