I missed the part about you having 2 subs. Same answer, just use the Main 2 outputs, connect one to Main R, the other to Main L. This is your answer.
I have 4 complete systems, all have subwoofers. One system has an ARC LS 26, similar outputs (but not as nice!) as your Ref 10. I have 4 subs in my main system, two of them are Rythmik F25s.