Martin Logan Purity vs Source

I understand the internal power and woofer differences between the Source and Purity, can you help me with the difference in sound. I intend on using in a 5.1 set up with a sub woofer and the ML Matinee center. Possibly Definitive Technology surrounds. Are Purity worth the extra $? If I choose the Purity can I save on the amp/pre amp set up?