I have only limited experience with the MX200 (friend of a friend owns it, I've heard it a few times in his system) but I think it is excellent.
The predecessor MX100 used an ESS DAC solution if I remember correctly, and it was very transparent and clean. They don't give us details about the DAC part on board the MX200 but I bet it is similarly excellent.
Now, you can totally use an outboard DAC if you wanted to achieve something different for stereo playback. Maybe a warmer, smoother, more forgiving sound for example, if that would fit your system's tuning better. But the MX200 should already be a really high caliber performer with a neutral, accurate presentation. So unless you have a specific goal, I'd stick with it at least for a while.