Compared to the M50x, the K553 sounds more open and expansive. It also has more engaging mids, where I find the M50x to be a little withdrawn. M50x is still an enjoyable headphone, but I prefer the K553 by no small margin.
I haven't heard MSR7, sorry.
It's been so long since I've heard HD598, I can't comment on that either.
HD600 is a better headphone - it's in a higher class. It sounds a little smoother/darker so if sibilance or bright recordings bother you,...