I dont run a sub as I run speakers that have usable bass to 16HZ but I may try to add my sub to fill in suck out if nothing else works
If your speakers are very symmetrically placed with respect to the room and listener (as most people) then you can assume they are exciting room modes in much the same way. A sub off to one side will also suffer from the same room modes - no doubt - so you will still have problems but not in quite the same way - especially if the sub is closer to the listening position than the speakers and at a different distance from floor and walls - so the combined effect of the mixing of the bass signals may help a little. Again the abundance of bass means you can use EQ to tame peaks which should better help you hear the troughs - just don't expect miracles as room modes are there whatever you do and they always dominate unless a room is extensively treated acoustically. Since you have bass down to 16 Hz then you can think of the subs role as "fill-in" duty...