If you can't find specifications of phones on the internet, then try GSM Arena - it's quite a good site for comparisons.
Here's their M9 review, for example:
http://www.gsmarena.com/htc_one_m9-review-1230.php
In the real world, you coluld also take your headphones into a store and try the handsets out.
Plus, why do you want to be lugging huge high impedence cans around with you in a portable scenario, where all the sonic benefits will be mitigated by environment and you'll look a bit, well, silly wearing them? This is where IEMs come into their own, given they are designed to work in this exactly this context.
Don't forget also, with Android phones, you can root and mod their architecture to tweak the sonics and amp gain in most cases. And even with iPhone, there are tips and solutions to help enhance the out of box experience.
Look, I'm not against those who want to spend mad money on esoteric grown up lego kits, but today's smartphones do a pretty bang up job of being the tech equivalent of swiss army knifes - great for most requirements on the go, including audio.