Dargo-Harry;
Yes, use the center/sub; Daisy chain out from the center to the new sub, just like you do now for Mains L/R... Also connect the Comms on the sub according to the manual;

Yes - MRC plays a big part - that's one of the advantagees in using an M- DSP sub with the MRC; Does a great job finding the peaks and dip as crion indicates. This is another effective tool that may help offset some of the advantages of 2 subs; although 2 will always be better, just maybe not enough better to justify it for some...

ken