No not grainy at all to my ear. A bit brighter, no question, but just as clear & clean as the Senns I think. But not too bright at all imo. Just enough to be a nice change of scenery -- sometimes I reach for the 99C just because. But no, they do not have the smoothness of the 600.
But you have to either be into, or indifferent about, their relatively big low end. I sometimes am into it, especially outdoors, where lows tend to get eaten up. And also at very low volumes. I'm only stressung this point (even though you didn't ask) because we're comparing against the 600, and the 99 have significantly more low end. (I personally find the 600 too light on bass most if the time.)
Anyway, I am really glad to have them both. If either were lost or damaged I'd replace them immediately!