Well if size doesn't matter then that's one factor to put in the back burner. The K550 is a sparkly headphone IMO. I'm sorry if i can't be clear on this but that is probably how i would describe them. Its not treble heavy or harsh sounding, it just sparkles. The soundstage is probably the widest of the listed, but i haven't heard any of the Audio Techninca's so i might be wrong. They are also pretty easy to drive.
Anyway that's just my opinion.
The Byer Dt 880 was bright n Sparkly so that sounds good
The Dt 990 [which I still own] has a nice big sound stage, as does my w1000x [especially after mods]
So, it should work for me. And my w1000x are NOT portable because they cost me to much! And I don't want to scratch that head band any more. And they can be finnicky to fit mostly because of thier sheer size and weight.
Still it took me a week to perfect the fit of the w1000x, takes me like 5 seconds, and a few seconds to readjust. I usually run the pads around the back of my ears then bring them forward a touch, give them a nice Snug to my head and slowy remove my hands a little. Which seems to work. The circlular motion seemed to get me the best fit. Make a circle around my ears then press forward, and presto!
And yea the w1000x is also very easy to drive, although I will be using Line out of my DAP so an amp is nessicary xD