You have hit the nail on the head here. Fully balanced through the Eximus is not as good (for me) as dual mono through the Naim. There was also a massive improvement in sound stage moving from one CMA800R to two in dual mono. The bass especially is incredibly powerful but not overbearing (although part of that is down to the Naim). I would never have thought it was possible to get such good, strong bass extension through HD800's.
It is also an excellent upgrade path as well.
I will be sticking with what I have for the time being, until the right preamp/DAC/balanced combination comes along to match the Questyle (I have already suggested to them that it is something they need to consider).