The reason I suggest it is that open headphones generally have a larger, more immersive soundstage. And the M50s are known for not having a good soundstage for a closed headphone, and you would definitely enjoy the difference. And at this moment, the Beyerdynamic DT990 Pro 250 Ohm are only $159 on Amazon (MSRP $299). They do need an amp, but you would have plenty in your budget for that and still have money left over.
100% agree with him!
Hey OP! Forgot about your portability concern, I misread it yesterday I guess! If you will be using it for portable, maybe you should cut the AKG K550 out of your list as well as the PSB M4U1 if you are afraid that it will fell down somewhere while on the go.
For portability closed headphones, I suggest you to look into the Sennheiser Momentum and MDR-1R. Both are great headphone with the Momentum having a very luxry build quality and design. The momentum is more natural sounding while the 1R is more colored. The 1R will have wider soundstage and tighter bass but does not extends as low as Momentum xD