I can only speak for the Q701 and Annies, since I owned a very old K701 that sounded a bit different from the Q701, and has gone through an extra bump, pad, and sonic changes, and I haven't heard a standard K702.
As for K612 and X1, i haven't heard the K612, but I'm betting it's a bit more polite, and less bassy than the X1. Going by graphs, both have a very clean/balanced mid to treble range, with the X1 having a peak at 10khz that isn't offensive at all to my ears, certainly no DT990 type peak. The X1 will be easy to drive, and i hear the K612 demands some decent amping. Willing to be the K612 is incredibly comfortable, if going by how the Annie with Q701 pads feels. The X1 is excellent in comfort.
Nokami, AD700x if you really need footstep whoring.
I want to get the best out of the X1 with my laptop, AND TB DSS. So what amp/DAC do you recommend for the X1 for cleaner bass?