that's a big room. i would probably call some of the manufactures of subs and get their input.some manufactures only suggest one sub for peak performance ex: REL SUBWOOFERS. i agree with you looking used as there are great deals to be had on this site that dealers cannot even come close to. as far as your rear channel i probably would ask totem what they think and would recommend. i think you are always better to use the same speaker manufacture for fronts,center and rear. the sub is a different story and to small a sub can be a real issue with the size of your room. with your dimensions i will tell you to rule out anything from a REL STADIUM or below as my room is close to yours and REL told me that this sub was to small and not to use two to make up the difference. i hope this helps you out.
bill
bill