I had the Dragonfly and the HD650s and while it was good, it's nothing on the Crack simply in terms of the dynamics created by the extra power referred to by PB above. The Crack will bring more energy, more life and more clarity to your HD650s (based on my experiences).
Also, while the DF is an excellent portable DAC, it's stage size is a little cramped so you may find a DAC upgrade after getting the Crack will take you forward yet another step. In the short term though, definitely go for the Crack - it'll be an excellent upgrade for your DF / HD650 combo