I'm sure that it'll sound better than the plain notebook audio system or an onboard soundcard. The Modi/Crack combintion is reasonably priced kit. Are you building the crack and getting the Speedball upgrade? I might prefer a Magni amp, but in the end if your ears are happy, that's what counts.
Yes, Modi/Crack, isn't expensive and good combo for the HD600 I guess. I'll build it myself, but no Speedball upgrade (maybe later). I read many good reviews (without Speedball) about the Crack (they say tubes sound kind of "liquid" and more natural) and it also looks nice :) I'll post a mini review about how the dedicated equipment vs. onboard soundcard performs. If I get a 2i2 or UR22 DAW (after Modi, for another PC) I'll compare them too.