As a starting point for this response, the 1,800 J is the impact energy that your drop tower is capable of reaching. Selection of which tup to use does not have as much to do with the impact energy as it does with the way that the sample being hit reacts to the impact.
For example, if a sample is made from a piece of tissue paper – I can impact that with 1800 J of energy and the load produced from the impact will be in the 10’s of foot-lbs. However, I can then insert a piece of ductile steel of the same dimensions, impact it with the same 1800 J and reach a load in the 200,000 ft-lbs. Both samples have been subjected to the same impact event – however, each reacts differently. So proper selection of a tup is more a factor of the material being tested and the anticipated maximum load from the impact event.