Hey Carl -
I promised I'd do some A/Bs with the gear once I finally got my JH13s back. I finally got them back last week, but I'm incredibly behind in my work, so I just some quick tests and will give you my very quick, initial take. So take with a grain of salt everyone, these are neither exhaustive nor conclusive in any way.
onboard soundcard of macbookpro 15in 2.2ghz headphone out
audio-gd sparrow (a-/b+ less than an A, more than a B - made specifically for JH13s) with a good (though cheap) optical connection to macbookpro
ibasso D4 with 9.6v battery and hiflight topkit
old, weary Senn 580s
Skipping test methodology, etc, etc.
Carl is vindicated! ;) Through my admittedly old and falling apart HD580s, the sound of the Sparrow and the mac were the closest of all in my tests. I could barely tell them apart, and certainly could certainly not reliably discern them blind. In trying to figure out why this is the case, I believe it is because the mac has a good amount of power output, and it has a "smooth" sound signature (like ipods) that mellows out harshness (at the cost of some detail). I could tell the computer and the D4 much more easily, because there was a definate harshness and angularity to the sound - more detail, but slightly less pleasurable to listen to. But overall, both amps and the computer were very, very similar through the 580s. Blind, I could not reliably tell them apart. When looking and trying to discern differences, I could tell minor differences and have a bunch of notes, but for the purposes of this, I'll just say they were quite small.
For the JH13s, it was a whole different story. First off, I could always instantly tell when the JH13s were plugged into the computer, because there was an unholy amount of line noise. Just lots of ugly static that neither of the amps had. Second, the computer was FAR too loud - I needed to turn everything way down to compare, which probably effected the sound a bit. The more interesting comparison then became between the sparrow and the D4. And honestly, here it was very hard to choose. Blind, it would totally depend on the music. There were certain tunes that I learned to hear the differences between them, while others I had no idea. Overall, it was impossible to reliably tell a difference between the two blind. But since I needed to sell one and keep the other, I had a very personal reason to decide which I felt was better / liked more. The D4 was about $100 more expensive with the accessories, but that wasn't a huge amount of money, and it was portable. Again, I won't belabor all the individual tests, songs, methodologies, details, etc. The general conclusion, for the JH13s, for me, was this: both amps sound better than the iphone 3gs, but not by so much that I'd want to carry a separate amp and LOD around with me on the subway - esp given the noise on the subway. Yes the Jh13s filter out the noise, but not 100%, and you have to have absolute quiet if you're going to hear the details that separate them. So the portability became a non-issue. Then it was just SQ. And honestly, I think both amps are fine. Neither was a runaway favorite. But finally, finally, I went with the customized Sparrow for two reasons: 1) the sound had all of the fine details, but a little bit more smoothness and refinement that made it more pleasurable to listen to at higher volumes. For classical, especially, there are sections where the D4 would have the violins shreeking up loud - the sparrow never did, they were present, detailed, but smoother. 2) the bass hump. I realized, after a lot of A/Bing, that the D4 and the sparrow both have a bass hump, but at different frequencies. The D4's is at the normal place for the subbass of a lot of pop/rap music. So if you have a song like Kanye West's Celebration, his bass drum 4/4 is right on the hump and gets grossly exaggerated. You don't hear it on every song, but it's definately there, and on some it sounds great, on some it's too much. The Sparrow has one that's lower down - at the very very bottom of the spectrum. Rather than a hump, it sounds more like 'fullness' or 'depth' - just a bit more 'oomph' from the music as a whole in the lower end, rather than a specific frequency. There were a few times where I thought the sparrow might be making things a bit muddier than the D4, but the majority of the time, I really liked this greater "solidity" to the bottom end. It made it more visceral - probably because of the increased power from the outlet.
So that's all - in the end, very very similar, minor differences. Apple obviously cares a lot about audio, and since they have a million audio people working on powerbooks and the majority of the market in ipods, they obviously know what they're doing and can produce good quality sound at a reasonable price. So do many good sound card manufacturers. Sparrow and D4 are similarly priced Dac/amps, with the D4/topkit a bit more for its portability and a bit less power due to its battery. Basically, exactly what you'd expect. As I've said before, and most people here would surely agree, as you spend more and more, you're going to get less and less for your money. The best value is probably whatever costs $20 these days, and it's all downhill from there. Nevertheless, I'm really happy with my JH13 / Sparrow / Iphone3gs combination, and since I got the Jh13 for about half off, I consider them a decent value - definitely a few steps above the Triple Fi's I had before.
Edited by AVU - 7/4/10 at 3:33pm