I think that the difference between the Rega DAC and Rega Apollo-R is the number of Wolfson WM8742 chips in them. I believe that the new Apollo-R uses one chip and the outboard DAC uses 2 Chips in differential mode (like the Saturn uses).
I would say add the Rega DAC to your Apollo. This should improve the sound.
I would say add the Rega DAC to your Apollo. This should improve the sound.