Here are a couple K612 impressions:
The K612 is more neutral than the K712, but not as neutral as the Q701 (going off memory).
Signature probably isn't that different than the HD600.
When you put the K712 pads on the K612, it becomes warmer than the K712. So the K612 drivers themselves are actually a little warmer sounding.
The K612 is really hungry for volume.
Midrange is a bit more forward on the K612 (compared to K712).
Compared to K712, it's...