I also found the opposite regarding amp power. I own the 4 and the 64gb 5S but my 5S is noticeably more powerful. (about 2 clicks) Maybe it has something to do with the different headphones we are using. I tried mine with IE80's & M100.
I do agree with the output impedance though. Unfortunately there are clear signs of it with my IE80's. More bass, and less clear with less detail. With higher impedance cans that goes away and it sounds better than my 4. With the IE80's it would be a toss up between the 4 despite the improvements with the 5S simply because of the impedance issue. A high impedance iem would sound wicked with it I would expect.
Nope, iPhone 4 and 5S have 16 volume steps. Go 8 steps on both and compare, it it obvious, iPhone 5S has weaker amp than iPhone 4. Other than that it is just awesome phone.
Edited by Jakkal - 10/8/13 at 3:29pm