When it comes to summing boxes ive allways found that, if everything is set flat with no analog eq's or compressors in the chain then the most noticeable difference will come from the flavour of the amps used in the summing box. There will also be a bit more separation and perhaps a better sense of space between everything depending on how you choose to group things, but this is going to be a subtle difference especially if your summing box and converters are really transparent.
If you can directly AB your summing box analog output with the digital print at the same listening level and hear a difference you don't like; smearing of transients ect.. then you know it is down to the conversion. Its quite a common phenomenon, also the reason why mastering engineers and mixers go all out on dedicated high end converters and clocks.
When it comes to most multi channel intefaces like the Ensemble or Symphony there is allways going to be a compromise between making something really good and making it affordable at the same time. Much in the same way that even high end dedicated converters can benefit from the use of external clocks.
However personally id first look at getting a better converter for printing the main mix: Burl, Antelope, Mytek, Lavry, or Cranesong. These will also have their own internal clock that you can sync with your Apogee. Then and only then can you intuitively decide if adding an external clock makes your system sound even better.