Forget KBB numbers ... the only way to tell what your car is worth is to actually sit down and SERIOUSLY negotiate for the new car. Where dealers start at means nothing ... where dealers END at and the out-money is all that matters.
Of course ... if you're so far upside down that you can never get out that's another matter. Learn to live with the car you have or go even FURTHER upside down by financing your negative equity into the new car. Not exactly a good financial idea, but a lot of people do it.