I had Z1 Xperia and found that Android has an inbuilt "safety" loudness limit that wouldn't even crank up my former cx300ii sennheisers, while iPhone had a lot bette output. Comparing it to iPod 5.5 it was of lesser quality. Having FiiO X5 which is like double the quality in the DAC and rocks with HD600, wouldn't trade for a phone honestly. And I won't be strapping any additional gear unless I get HD800S eventually.
Well that's the thing, most people would use the phone to stream audio and then use an IEM, which won't drive decent smartphones into audible distortion (save for some kind of safety feature on EU phones, but then again, given the sensitivity level of the IEMs, even that won't keep it from getting loud), for use in a public place. Once we get around to open back headphones it's at home and there are other ways to use it, and most people prefer their computers or laptops. Or even with DAPs, but the thing is, in any case there are a lot of alternatives to a dedicated DAP with phone functions, and those computers/laptops can access lossless streaming.