I've skimmed through this thread, so forgive my ignorance if I missed it. Has anyone properly analyzed the audio on the 5S? It's been stated it has a new chip/amp so it should yield different measurements from what kenrockwell recorded.
I measured the output impedance at 2.30 ohms which is significantly better than the 5. I also measured max voltage out into 100 ohms and the 5S was slightly higher than the 4, so it's got about as much power as the 4, which is--from what I read--a step up from the 5. And there was one site posted earlier that did some measurements when driving a specific headphone (FR graph)--an Audio Technica I think--but so far I don't know of any extensive testing and measuring like Ken Rockwell did for the 5.
But the general consensus from around the web is that the 5S is a step up in SQ from the 5, for sure, but whether or not it is as good as the 4S or 4 hasn't reached consensus. I find it to be a step up in SQ from my iPhone 4, but not with my Senn PX-100's. I suspect it might have something to do with the impedance of the headphones involved. Anecdotally I have found so far that the 5S likes higher impedance phones (50+ ohms) that have a nice high sensitivity. Although I find the Ety ER-4S (100 ohms and medium sensitivity) to sound great out of the 5S.
I have a JDS Labs C5D to compare against, and while I'm still in the "impressions" stage, my impression is that with the ER-4S's, the 5S scores about an 85-90 if we count the C5D as a 100. Which is pretty impressive. The Ety's sound better out of the C5D, especially in the treble, but not so much better that I sometimes don't just decide it's not worth the bother of hooking up the C5D, and just listen straight out of the 5S. And love it.
I think if I had to characterize the 5S strengths and flaws at this point I would say that the bass CAN suffer on lower impedance cans (my Grado SR225's and PX100's) and that it CAN sound a bit harsh and edgy and forward with certain headphones (my Sony MDR-7506's and Grado SR-225's) and in any case doesn't have the most smooth and liquid of treble's,
BUT
it has noticeably better detail and clarity than the iPhone 4, which sounds a bit "soft" and polite when I go back and listen to it.
Overall, I find it to be a win, but it does depend on what headphone you pair it with.
Right now, I really like my ER-4S's and Sennheiser HD595's straight out of the 5S. With these two phones, the 5S is CLEARLY better in SQ than the 4.