I tested k550 today. i lost all interest in them. they are the best detailed, best treble, best bass, best just BEST at 300$ price point. Period. But i am going to get something around 1000$ or more price point, and k550 is not in the same league. anyways, if anyone needs the best headphone under 300$, and it has to be neutral, this is the safest bet.
my ultrasone dj one pro sound much much better for me, but they are not neutral, they are sweet and fun. k550 was much more pointing towards neutral.
depending on my mood, if i were blocked on a 300$ budget i would either get k550, ultrasone dj one pro, or sennheiser urbanite xl, but k550 was the most neutral, out of all. the timbre was nice. out of fiio x5 was a little under-powered. adding fiio e12 added so much more exactity to the sound.