Putting mb quartz in a system with Dynaudios is akin to filling a comfortable leather chair with needles and razorblades.
You probably couldn't find two more opposite speakers.
Why do you want rears? they will just screw up your stage and add more cancellation into the mix.
If you MUST do it, I'd suggest finding just a simple midrange, no tweeter, and running it from the head unit. This will "fill in your sound" if thats what you are looking for, and it will screw up your soundstage much less than a coaxial.
A pair of these:
http://www.madisound.com/catalog/pro...oducts_id=8188
I'm not sure what the mounting diameter and depth is in a c5 though.