Exactly my point. I have tried $1000-2000 dacs as well and although in a blind test some people can tell them from an iphone, the difference isn't night and day and is really close. And I myself own a $900 dac which is fairly close to the iphone though a bit better but not as flat or neutral but sounds slightly better with my hd 650, and is a bit smoother. But the differences are less than 1-2% tops.
The udac2 used to measure worse than the iphone dac and most dacs under $200 won't be different from an iphone dac or worse. The only which are comparable or slightly better are likely odac and asus stx and perhaps a few more.
Dacs which you can tell apart from the above you can only do so because they aren't as good. They may sound decent with some kit but won't be as neutral or flat as the above stuff. I would buy an odac or asus stx over a nuforce or modi. If you can go for a bifrost then that should be okay as well. But don't expect miracles. It could be a bit less harsh and smoother than cheaper dacs and that is it.
I used a FiiO E12 with my ipad dac for HD 598, the iphone headphone out is better quality sound. The FiiO E12 is much louder, slightly smoother and much warmer with better bass for most. But the iphone or ipad headphone out is closer to neutral and not loud enough and runs out of steam way before you realize. But had the iphone headphone out been much much louder with enough headroom i would prefer it to fiio e12.
And i own all this stuff including expensive dacs too.
Most people get confused between volume levels and lack of headroom. Most gear runs out of steam pretty quickly and isn't usable in a lot of stuff. but that doesn't mean it is technically worse, it just needs the right partnering equipment. Same with most passive stereo amps in the sub $2000-3000 range, most of them clip at regular volumes which nobody realizes and hence sound different.
And this is when the iphone headphone out is its weakeness, but if it had more headroom and volume I would take it over the Fiio E12, I don't need a warmer sound. I need a clearer sound.