Did they tell you what color the 600 ohm drivers are? I just got some 600 ohm drivers from Beyer which had stickers on them, and so I am sure they are 600 ohm - so I could check the cable color to see if that's uniform.
For what its worth, my recabled (fat pipe) 250 ohm is still my fave over the new 600 ohm version. The latter came with the long stock cable. But I'm only at about 50 hours (about 45 with a burn-in track on repeat; the balance on my head this week). I feel there's a boomier bass on the 600ohm version, and slightly less space in the soundstage. However, I'm hoping this will all catch up over time.