How much placement flexibility do you have. That would be the best scenario to minimize them as much as possible using placement. You see multiple subs touted as the cure but only over the subs operating range. Unfortunately it's fairly common to have them above sub frequencies in small rooms.
The nulls you are stuck with so if you can minimize the number of peaks and severity of them using placement you are off to a good start. At this point you could try bass traps if you have room or use selective high Q notch filters, DSP, to get them minimized as best you can.
Good luck
Rob