Take with a grain of salt - I find that SR80i have slightly more mid-bass punch and Ety have bit more low-bass presence (looking at the Golden Ears measurments it shouldn't be the case really, but I'll pick HF5 over any day for something like...

Well, source output impedance indeed affects the way headphones sound, however it's not your case as FiiO X3 rated less than 0.3 ohm. I believe it's perfectly fine for SE846 (note I don't own them or X3) :3

  Going opposite :D So, to mess things up for you even more I'd add to the list Audio-Technica ATH-IM70/ATH-IM50 Dunu DN-1000, these are out of your budget, though there's an option picking them for less on [Mod Edit: We don't allow discussion of that site.] Also, if you haven't read this...