Soundstage. That is the only word you need to know when considering K550s. The best in a sealed headphone I've ever heard. If Sennheiser's put you in the fifth row, these put you on the stage. For me, the alpha and the omega of why I love my K550s is the soundstage, period.
As for the K702 comparison...meh. The K550s are a balanced headphone. The highs roll off. There is mid-high spike that I equalize down, other than that, these cans favor the left-center of the frequency spectrum. I find the bass the tiniest bit understated, so i equalize up 60-200 Hz just a tad. So in short, they sound nothing like the K702s...to me.