Best headphone amp for classical music CDs?

I am listening to classical music CDs on Sennheiser HD700s through the headphone jack of the Marantz CD6006. I am pretty happy with the sound--the built-in DAC/headphone amp on the Marantz are good. Question: would a separate headphone amp (not DAC) make it better? In particular, I have been considering the Burson Audio Soloist SL MK2 and the Graham Slee Novo.

Please let me know which of these you think would be better or whether neither of these would really improve on the Marantz sound.