I own the 600's and borrowed my buddies 400's for a couple months. While the 400 does have stronger and punchier bass, I found the high range a bit honky. Especially with vocals. There is a hollowness to the sound. I have found this trait in all the ortho cans. I have spent a lot of time with the he500, 400, and the LCD 2 and 3. I won't own any of them because of this trait. They are great cans, but what is important to me isn't good enough. One exception is the LCD X. I spent a little time with it at a recent meet. It is very good. Hoping to try it again soon.
So. The 400 is a good value for how it sounds. But for me, it's just not as well balanced as the 600.