CeteaThe K7xx is open while the M50(x)s is closed, which mean two completely different sound stages, also, I didn't like the bass (metal only playlist) when I tried them, they were really "bright" to me.
But I haven't tried these yet, just giving my very simple non audiophile opinion on the M50(x)'s.
VeRKKThe difference between open or closed isn't the soundstage. Open backed headphones will sound different (usually for the better) but at the cost of sound isolation. Open backed headphones are like anti-noise-cancelling headphones.