Mike. Yes I have the 600s connected at line level L and R large signal from the processors XLR outputs, then crossing at the subs. The LFE plus redirected surrounds from the processor and unbalanced RCA large L and R are run through the SMS and crossed. Then the speaker signals output to the amp. I thought I may have had a problem with cascading crossovers for the surrounds but that doesn't appear to be the case.
Your suggestion of using separate RCA L and R into each L and R sub should work as well.
St. Pat. Well put. I alternate between thinking its either a curse or a blessing. If you ask my wife she would suggest I'm in the former category based on the time and $$s spent.