It's good to hear that most people are liking the SQ of the 5S, but, of course, bad to hear that there is an issue with the output power!
Personally, I have still not yet had a time when I couldn't get all the volume I wanted / needed out of the ER-4S's and iP5S combo. I even have a volume limit programmed in so I don't accidentally blast my ears.
As for the iP4 vs. iP5S, I loved my 4 and found it to be very musical and enjoyable, but going back and forth between the two, I finally found myself preferring the 5S by a good margin with my preferred headphones. No contest with the Ety ER-4S's--clearly better on the 5S, but yes, definitely not lacking in the high end and detail and some might find this too "aggressive" or harsh. Not me, though. I love the detail, clarity, and resolution, and I don't think that it is emphasizing the highs at all. Not that my ears can tell, anyway. And I definitely don't find the ER-4S's to be "thin" sounding out of the 5S. I marvel at the SQ and easily get lost in the music. It's a detail monster, to be sure, but not thin.
And with my HD-595's, again no contest--noticeably better out of the 5S. The PX-100's, however, were definitely much better out of the 4.
Anyway, I can totally sympathize with those of you worried about the SQ and output power of the 5S. For me as well, music is one of the most important things that I do with my iPhone and I'm not keen on using an outboard amp or amp+dac. It's decidedly inconvenient.