From my experience, a high contact point is very critical. It is common sense that tha higher the ball drops from the higher it will rebound. This also allows for the ball to drop at a steeper angle, and when that is combined with the increased leverage of a higher contact point, you get more kick. I have noticed that the pros who contact the higher and further into the court, typically have the biggest serves. Being further into the court also reduces the distance the ball has to travel from the racquet to the ground, and that reduces the air resistance that ball will experience, which leads to more speed being upon striking the ground.
If you want more kick without adjustments to your technique you can always use a racquet with fewer mains, and more lively string like NXT 17 will help. Sampras had a great kick serve, and he used a 16 mains racquet with 17 gauge gut. I don't believe I am going to say this, but you can always use the Babolat frames to give you extra kick.